Re: [PATCH v2 02/20] kvm: x86/mmu: Introduce tdp_iter

From: Yu Zhang
Date: Wed Oct 21 2020 - 22:34:49 EST

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On Wed, Oct 21, 2020 at 11:08:52AM -0700, Ben Gardon wrote:
> On Wed, Oct 21, 2020 at 7:59 AM Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> >
> > On Wed, Oct 14, 2020 at 11:26:42AM -0700, Ben Gardon wrote:
> > > The TDP iterator implements a pre-order traversal of a TDP paging
> > > structure. This iterator will be used in future patches to create
> > > an efficient implementation of the KVM MMU for the TDP case.
> > >
> > > Tested by running kvm-unit-tests and KVM selftests on an Intel Haswell
> > > machine. This series introduced no new failures.
> > >
> > > This series can be viewed in Gerrit at:
> > > https://linux-review.googlesource.com/c/virt/kvm/kvm/+/2538
> > >
> > > Signed-off-by: Ben Gardon <bgardon@xxxxxxxxxx>
> > > ---
> > > arch/x86/kvm/Makefile | 3 +-
> > > arch/x86/kvm/mmu/mmu.c | 66 ------------
> > > arch/x86/kvm/mmu/mmu_internal.h | 66 ++++++++++++
> > > arch/x86/kvm/mmu/tdp_iter.c | 176 ++++++++++++++++++++++++++++++++
> > > arch/x86/kvm/mmu/tdp_iter.h | 56 ++++++++++
> > > 5 files changed, 300 insertions(+), 67 deletions(-)
> > > create mode 100644 arch/x86/kvm/mmu/tdp_iter.c
> > > create mode 100644 arch/x86/kvm/mmu/tdp_iter.h
> > >
> > > diff --git a/arch/x86/kvm/Makefile b/arch/x86/kvm/Makefile
> > > index 7f86a14aed0e9..4525c1151bf99 100644
> > > --- a/arch/x86/kvm/Makefile
> > > +++ b/arch/x86/kvm/Makefile
> > > @@ -15,7 +15,8 @@ kvm-$(CONFIG_KVM_ASYNC_PF) += $(KVM)/async_pf.o
> > >
> > > kvm-y += x86.o emulate.o i8259.o irq.o lapic.o \
> > > i8254.o ioapic.o irq_comm.o cpuid.o pmu.o mtrr.o \
> > > - hyperv.o debugfs.o mmu/mmu.o mmu/page_track.o
> > > + hyperv.o debugfs.o mmu/mmu.o mmu/page_track.o \
> > > + mmu/tdp_iter.o
> > >
> > > kvm-intel-y += vmx/vmx.o vmx/vmenter.o vmx/pmu_intel.o vmx/vmcs12.o \
> > > vmx/evmcs.o vmx/nested.o vmx/posted_intr.o
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index 6c9db349600c8..6d82784ed5679 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -121,28 +121,6 @@ module_param(dbg, bool, 0644);
> > >
> > > #define PTE_PREFETCH_NUM 8
> > >
> > > -#define PT_FIRST_AVAIL_BITS_SHIFT 10
> > > -#define PT64_SECOND_AVAIL_BITS_SHIFT 54
> > > -
> > > -/*
> > > - * The mask used to denote special SPTEs, which can be either MMIO SPTEs or
> > > - * Access Tracking SPTEs.
> > > - */
> > > -#define SPTE_SPECIAL_MASK (3ULL << 52)
> > > -#define SPTE_AD_ENABLED_MASK (0ULL << 52)
> > > -#define SPTE_AD_DISABLED_MASK (1ULL << 52)
> > > -#define SPTE_AD_WRPROT_ONLY_MASK (2ULL << 52)
> > > -#define SPTE_MMIO_MASK (3ULL << 52)
> > > -
> > > -#define PT64_LEVEL_BITS 9
> > > -
> > > -#define PT64_LEVEL_SHIFT(level) \
> > > - (PAGE_SHIFT + (level - 1) * PT64_LEVEL_BITS)
> > > -
> > > -#define PT64_INDEX(address, level)\
> > > - (((address) >> PT64_LEVEL_SHIFT(level)) & ((1 << PT64_LEVEL_BITS) - 1))
> > > -
> > > -
> > > #define PT32_LEVEL_BITS 10
> > >
> > > #define PT32_LEVEL_SHIFT(level) \
> > > @@ -155,19 +133,6 @@ module_param(dbg, bool, 0644);
> > > #define PT32_INDEX(address, level)\
> > > (((address) >> PT32_LEVEL_SHIFT(level)) & ((1 << PT32_LEVEL_BITS) - 1))
> > >
> > > -
> > > -#ifdef CONFIG_DYNAMIC_PHYSICAL_MASK
> > > -#define PT64_BASE_ADDR_MASK (physical_mask & ~(u64)(PAGE_SIZE-1))
> > > -#else
> > > -#define PT64_BASE_ADDR_MASK (((1ULL << 52) - 1) & ~(u64)(PAGE_SIZE-1))
> > > -#endif
> > > -#define PT64_LVL_ADDR_MASK(level) \
> > > - (PT64_BASE_ADDR_MASK & ~((1ULL << (PAGE_SHIFT + (((level) - 1) \
> > > - * PT64_LEVEL_BITS))) - 1))
> > > -#define PT64_LVL_OFFSET_MASK(level) \
> > > - (PT64_BASE_ADDR_MASK & ((1ULL << (PAGE_SHIFT + (((level) - 1) \
> > > - * PT64_LEVEL_BITS))) - 1))
> > > -
> > > #define PT32_BASE_ADDR_MASK PAGE_MASK
> > > #define PT32_DIR_BASE_ADDR_MASK \
> > > (PAGE_MASK & ~((1ULL << (PAGE_SHIFT + PT32_LEVEL_BITS)) - 1))
> > > @@ -192,8 +157,6 @@ module_param(dbg, bool, 0644);
> > > #define SPTE_HOST_WRITEABLE (1ULL << PT_FIRST_AVAIL_BITS_SHIFT)
> > > #define SPTE_MMU_WRITEABLE (1ULL << (PT_FIRST_AVAIL_BITS_SHIFT + 1))
> > >
> > > -#define SHADOW_PT_INDEX(addr, level) PT64_INDEX(addr, level)
> > > -
> > > /* make pte_list_desc fit well in cache line */
> > > #define PTE_LIST_EXT 3
> > >
> > > @@ -349,11 +312,6 @@ void kvm_mmu_set_mmio_spte_mask(u64 mmio_value, u64 access_mask)
> > > }
> > > EXPORT_SYMBOL_GPL(kvm_mmu_set_mmio_spte_mask);
> > >
> > > -static bool is_mmio_spte(u64 spte)
> > > -{
> > > - return (spte & SPTE_SPECIAL_MASK) == SPTE_MMIO_MASK;
> > > -}
> > > -
> > > static inline bool sp_ad_disabled(struct kvm_mmu_page *sp)
> > > {
> > > return sp->role.ad_disabled;
> > > @@ -626,35 +584,11 @@ static int is_nx(struct kvm_vcpu *vcpu)
> > > return vcpu->arch.efer & EFER_NX;
> > > }
> > >
> > > -static int is_shadow_present_pte(u64 pte)
> > > -{
> > > - return (pte != 0) && !is_mmio_spte(pte);
> > > -}
> > > -
> > > -static int is_large_pte(u64 pte)
> > > -{
> > > - return pte & PT_PAGE_SIZE_MASK;
> > > -}
> > > -
> > > -static int is_last_spte(u64 pte, int level)
> > > -{
> > > - if (level == PG_LEVEL_4K)
> > > - return 1;
> > > - if (is_large_pte(pte))
> > > - return 1;
> > > - return 0;
> > > -}
> > > -
> > > static bool is_executable_pte(u64 spte)
> > > {
> > > return (spte & (shadow_x_mask | shadow_nx_mask)) == shadow_x_mask;
> > > }
> > >
> > > -static kvm_pfn_t spte_to_pfn(u64 pte)
> > > -{
> > > - return (pte & PT64_BASE_ADDR_MASK) >> PAGE_SHIFT;
> > > -}
> > > -
> > > static gfn_t pse36_gfn_delta(u32 gpte)
> > > {
> > > int shift = 32 - PT32_DIR_PSE36_SHIFT - PAGE_SHIFT;
> > > diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> > > index 3acf3b8eb469d..74ccbf001a42e 100644
> > > --- a/arch/x86/kvm/mmu/mmu_internal.h
> > > +++ b/arch/x86/kvm/mmu/mmu_internal.h
> > > @@ -2,6 +2,8 @@
> > > #ifndef __KVM_X86_MMU_INTERNAL_H
> > > #define __KVM_X86_MMU_INTERNAL_H
> > >
> > > +#include "mmu.h"
> > > +
> > > #include <linux/types.h>
> > >
> > > #include <asm/kvm_host.h>
> > > @@ -60,4 +62,68 @@ void kvm_mmu_gfn_allow_lpage(struct kvm_memory_slot *slot, gfn_t gfn);
> > > bool kvm_mmu_slot_gfn_write_protect(struct kvm *kvm,
> > > struct kvm_memory_slot *slot, u64 gfn);
> > >
> > > +#define PT64_LEVEL_BITS 9
> > > +
> > > +#define PT64_LEVEL_SHIFT(level) \
> > > + (PAGE_SHIFT + (level - 1) * PT64_LEVEL_BITS)
> > > +
> > > +#define PT64_INDEX(address, level)\
> > > + (((address) >> PT64_LEVEL_SHIFT(level)) & ((1 << PT64_LEVEL_BITS) - 1))
> > > +#define SHADOW_PT_INDEX(addr, level) PT64_INDEX(addr, level)
> > > +
> > > +#define PT_FIRST_AVAIL_BITS_SHIFT 10
> > > +#define PT64_SECOND_AVAIL_BITS_SHIFT 54
> > > +
> > > +/*
> > > + * The mask used to denote special SPTEs, which can be either MMIO SPTEs or
> > > + * Access Tracking SPTEs.
> > > + */
> > > +#define SPTE_SPECIAL_MASK (3ULL << 52)
> > > +#define SPTE_AD_ENABLED_MASK (0ULL << 52)
> > > +#define SPTE_AD_DISABLED_MASK (1ULL << 52)
> > > +#define SPTE_AD_WRPROT_ONLY_MASK (2ULL << 52)
> > > +#define SPTE_MMIO_MASK (3ULL << 52)
> > > +
> > > +#ifdef CONFIG_DYNAMIC_PHYSICAL_MASK
> > > +#define PT64_BASE_ADDR_MASK (physical_mask & ~(u64)(PAGE_SIZE-1))
> > > +#else
> > > +#define PT64_BASE_ADDR_MASK (((1ULL << 52) - 1) & ~(u64)(PAGE_SIZE-1))
> > > +#endif
> > > +#define PT64_LVL_ADDR_MASK(level) \
> > > + (PT64_BASE_ADDR_MASK & ~((1ULL << (PAGE_SHIFT + (((level) - 1) \
> > > + * PT64_LEVEL_BITS))) - 1))
> > > +#define PT64_LVL_OFFSET_MASK(level) \
> > > + (PT64_BASE_ADDR_MASK & ((1ULL << (PAGE_SHIFT + (((level) - 1) \
> > > + * PT64_LEVEL_BITS))) - 1))
> > > +
> > > +/* Functions for interpreting SPTEs */
> > > +static inline bool is_mmio_spte(u64 spte)
> > > +{
> > > + return (spte & SPTE_SPECIAL_MASK) == SPTE_MMIO_MASK;
> > > +}
> > > +
> > > +static inline int is_shadow_present_pte(u64 pte)
> > > +{
> > > + return (pte != 0) && !is_mmio_spte(pte);
> > > +}
> > > +
> > > +static inline int is_large_pte(u64 pte)
> > > +{
> > > + return pte & PT_PAGE_SIZE_MASK;
> > > +}
> > > +
> > > +static inline int is_last_spte(u64 pte, int level)
> > > +{
> > > + if (level == PG_LEVEL_4K)
> > > + return 1;
> > > + if (is_large_pte(pte))
> > > + return 1;
> > > + return 0;
> > > +}
> > > +
> > > +static inline kvm_pfn_t spte_to_pfn(u64 pte)
> > > +{
> > > + return (pte & PT64_BASE_ADDR_MASK) >> PAGE_SHIFT;
> > > +}
> > > +
> > > #endif /* __KVM_X86_MMU_INTERNAL_H */
> > > diff --git a/arch/x86/kvm/mmu/tdp_iter.c b/arch/x86/kvm/mmu/tdp_iter.c
> > > new file mode 100644
> > > index 0000000000000..b07e9f0c5d4aa
> > > --- /dev/null
> > > +++ b/arch/x86/kvm/mmu/tdp_iter.c
> > > @@ -0,0 +1,176 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +
> > > +#include "mmu_internal.h"
> > > +#include "tdp_iter.h"
> > > +
> > > +/*
> > > + * Recalculates the pointer to the SPTE for the current GFN and level and
> > > + * reread the SPTE.
> > > + */
> > > +static void tdp_iter_refresh_sptep(struct tdp_iter *iter)
> > > +{
> > > + iter->sptep = iter->pt_path[iter->level - 1] +
> > > + SHADOW_PT_INDEX(iter->gfn << PAGE_SHIFT, iter->level);
> > > + iter->old_spte = READ_ONCE(*iter->sptep);
> > > +}
> > > +
> > > +static gfn_t round_gfn_for_level(gfn_t gfn, int level)
> > > +{
> > > + return gfn - (gfn % KVM_PAGES_PER_HPAGE(level));
> >
> > Instead of the modulo operator, how about we use:
> > return gfn & ~(KVM_PAGES_PER_HPAGE(level) - 1);
> > here? :)
> >
> > > +}
> > > +
> > > +/*
> > > + * Sets a TDP iterator to walk a pre-order traversal of the paging structure
> > > + * rooted at root_pt, starting with the walk to translate goal_gfn.
> > > + */
> > > +void tdp_iter_start(struct tdp_iter *iter, u64 *root_pt, int root_level,
> > > + int min_level, gfn_t goal_gfn)
> > > +{
> > > + WARN_ON(root_level < 1);
> > > + WARN_ON(root_level > PT64_ROOT_MAX_LEVEL);
> > > +
> > > + iter->goal_gfn = goal_gfn;
> > > + iter->root_level = root_level;
> > > + iter->min_level = min_level;
> > > + iter->level = root_level;
> > > + iter->pt_path[iter->level - 1] = root_pt;
> > > +
> > > + iter->gfn = round_gfn_for_level(iter->goal_gfn, iter->level);
> > > + tdp_iter_refresh_sptep(iter);
> > > +
> > > + iter->valid = true;
> > > +}
> > > +
> > > +/*
> > > + * Given an SPTE and its level, returns a pointer containing the host virtual
> > > + * address of the child page table referenced by the SPTE. Returns null if
> > > + * there is no such entry.
> > > + */
> > > +u64 *spte_to_child_pt(u64 spte, int level)
> > > +{
> > > + /*
> > > + * There's no child entry if this entry isn't present or is a
> > > + * last-level entry.
> > > + */
> > > + if (!is_shadow_present_pte(spte) || is_last_spte(spte, level))
> > > + return NULL;
> > > +
> > > + return __va(spte_to_pfn(spte) << PAGE_SHIFT);
> > > +}
> > > +
> > > +/*
> > > + * Steps down one level in the paging structure towards the goal GFN. Returns
> > > + * true if the iterator was able to step down a level, false otherwise.
> > > + */
> > > +static bool try_step_down(struct tdp_iter *iter)
> > > +{
> > > + u64 *child_pt;
> > > +
> > > + if (iter->level == iter->min_level)
> > > + return false;
> > > +
> > > + /*
> > > + * Reread the SPTE before stepping down to avoid traversing into page
> > > + * tables that are no longer linked from this entry.
> > > + */
> > > + iter->old_spte = READ_ONCE(*iter->sptep);
> > > +
> > > + child_pt = spte_to_child_pt(iter->old_spte, iter->level);
> > > + if (!child_pt)
> > > + return false;
> > > +
> > > + iter->level--;
> > > + iter->pt_path[iter->level - 1] = child_pt;
> > > + iter->gfn = round_gfn_for_level(iter->goal_gfn, iter->level);
> > > + tdp_iter_refresh_sptep(iter);
> > > +
> > > + return true;
> > > +}
> > > +
> > > +/*
> > > + * Steps to the next entry in the current page table, at the current page table
> > > + * level. The next entry could point to a page backing guest memory or another
> > > + * page table, or it could be non-present. Returns true if the iterator was
> > > + * able to step to the next entry in the page table, false if the iterator was
> > > + * already at the end of the current page table.
> > > + */
> > > +static bool try_step_side(struct tdp_iter *iter)
> > > +{
> > > + /*
> > > + * Check if the iterator is already at the end of the current page
> > > + * table.
> > > + */
> > > + if (!((iter->gfn + KVM_PAGES_PER_HPAGE(iter->level)) %
> > > + KVM_PAGES_PER_HPAGE(iter->level + 1)))
> > > + return false;
> > > +
> >
> > And maybe:
> > if (SHADOW_PT_INDEX(iter->gfn << PAGE_SHIFT, iter->level) ==
> > (PT64_ENT_PER_PAGE - 1))
> > here?
> >
> > > + iter->gfn += KVM_PAGES_PER_HPAGE(iter->level);
> > > + iter->goal_gfn = iter->gfn;
> > > + iter->sptep++;
> > > + iter->old_spte = READ_ONCE(*iter->sptep);
> > > +
> > > + return true;
> > > +}
> > > +
> > > +/*
> > > + * Tries to traverse back up a level in the paging structure so that the walk
> > > + * can continue from the next entry in the parent page table. Returns true on a
> > > + * successful step up, false if already in the root page.
> > > + */
> > > +static bool try_step_up(struct tdp_iter *iter)
> > > +{
> > > + if (iter->level == iter->root_level)
> > > + return false;
> > > +
> > > + iter->level++;
> > > + iter->gfn = round_gfn_for_level(iter->gfn, iter->level);
> > > + tdp_iter_refresh_sptep(iter);
> > > +
> > > + return true;
> > > +}
> > > +
> > > +/*
> > > + * Step to the next SPTE in a pre-order traversal of the paging structure.
> > > + * To get to the next SPTE, the iterator either steps down towards the goal
> > > + * GFN, if at a present, non-last-level SPTE, or over to a SPTE mapping a
> > > + * highter GFN.
> > > + *
> > > + * The basic algorithm is as follows:
> > > + * 1. If the current SPTE is a non-last-level SPTE, step down into the page
> > > + * table it points to.
> > > + * 2. If the iterator cannot step down, it will try to step to the next SPTE
> > > + * in the current page of the paging structure.
> > > + * 3. If the iterator cannot step to the next entry in the current page, it will
> > > + * try to step up to the parent paging structure page. In this case, that
> > > + * SPTE will have already been visited, and so the iterator must also step
> > > + * to the side again.
> > > + */
> > > +void tdp_iter_next(struct tdp_iter *iter)
> > > +{
> > > + if (try_step_down(iter))
> > > + return;
> > > +
> > > + do {
> > > + if (try_step_side(iter))
> > > + return;
> > > + } while (try_step_up(iter));
> > > + iter->valid = false;
> > > +}
> > > +
> > > +/*
> > > + * Restart the walk over the paging structure from the root, starting from the
> > > + * highest gfn the iterator had previously reached. Assumes that the entire
> > > + * paging structure, except the root page, may have been completely torn down
> > > + * and rebuilt.
> > > + */
> > > +void tdp_iter_refresh_walk(struct tdp_iter *iter)
> > > +{
> > > + gfn_t goal_gfn = iter->goal_gfn;
> > > +
> > > + if (iter->gfn > goal_gfn)
> > > + goal_gfn = iter->gfn;
> > > +
> > > + tdp_iter_start(iter, iter->pt_path[iter->root_level - 1],
> > > + iter->root_level, iter->min_level, goal_gfn);
> > > +}
> > > +
> > > diff --git a/arch/x86/kvm/mmu/tdp_iter.h b/arch/x86/kvm/mmu/tdp_iter.h
> > > new file mode 100644
> > > index 0000000000000..d629a53e1b73f
> > > --- /dev/null
> > > +++ b/arch/x86/kvm/mmu/tdp_iter.h
> > > @@ -0,0 +1,56 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +
> > > +#ifndef __KVM_X86_MMU_TDP_ITER_H
> > > +#define __KVM_X86_MMU_TDP_ITER_H
> > > +
> > > +#include <linux/kvm_host.h>
> > > +
> > > +#include "mmu.h"
> > > +
> > > +/*
> > > + * A TDP iterator performs a pre-order walk over a TDP paging structure.
> > > + */
> > > +struct tdp_iter {
> > > + /*
> > > + * The iterator will traverse the paging structure towards the mapping
> > > + * for this GFN.
> > > + */
> > > + gfn_t goal_gfn;
> > > + /* Pointers to the page tables traversed to reach the current SPTE */
> > > + u64 *pt_path[PT64_ROOT_MAX_LEVEL];
> > > + /* A pointer to the current SPTE */
> > > + u64 *sptep;
> > > + /* The lowest GFN mapped by the current SPTE */
> > > + gfn_t gfn;
> > > + /* The level of the root page given to the iterator */
> > > + int root_level;
> > > + /* The lowest level the iterator should traverse to */
> > > + int min_level;
> > > + /* The iterator's current level within the paging structure */
> > > + int level;
> > > + /* A snapshot of the value at sptep */
> > > + u64 old_spte;
> > > + /*
> > > + * Whether the iterator has a valid state. This will be false if the
> > > + * iterator walks off the end of the paging structure.
> > > + */
> > > + bool valid;
> > > +};
> > > +
> > > +/*
> > > + * Iterates over every SPTE mapping the GFN range [start, end) in a
> > > + * preorder traversal.
> > > + */
> > > +#define for_each_tdp_pte(iter, root, root_level, start, end) \
> > > + for (tdp_iter_start(&iter, root, root_level, PG_LEVEL_4K, start); \
> > > + iter.valid && iter.gfn < end; \
> > > + tdp_iter_next(&iter))
> > > +
> > > +u64 *spte_to_child_pt(u64 pte, int level);
> > > +
> > > +void tdp_iter_start(struct tdp_iter *iter, u64 *root_pt, int root_level,
> > > + int min_level, gfn_t goal_gfn);
> > > +void tdp_iter_next(struct tdp_iter *iter);
> > > +void tdp_iter_refresh_walk(struct tdp_iter *iter);
> > > +
> > > +#endif /* __KVM_X86_MMU_TDP_ITER_H */
> > > --
> > > 2.28.0.1011.ga647a8990f-goog
> > >
> >
> > I am just suggesting to replace the modulo operations with bit-shifts...
> > Also, it's very exciting to see such patch set. Thanks!
>
> Those are great suggestions, thank you. I wonder if the compiler would
> have been smart enough to convert those to bit shifts. (I kind of
> doubt it)
> I'm glad you're excited about this patch set, thank you for taking a
> look and helping to review it!
> It doesn't sound like I'll be sending out a v3 of this set, but I'll
> be sure to amend the code with your suggestions if they don't make it
> into the version Paolo queues.

Great. And looking forward to the performance improvement series! :)

Yu

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