Re: [RFC PATCH v2 41/69] KVM: x86: Add infrastructure for stolen GPA bits

From: Kai Huang
Date: Thu Aug 05 2021 - 07:44:34 EST

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On Fri, 2 Jul 2021 15:04:47 -0700 isaku.yamahata@xxxxxxxxx wrote:
> From: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
>
> Add support in KVM's MMU for aliasing multiple GPAs (from a hardware
> perspective) to a single GPA (from a memslot perspective). GPA alising
> will be used to repurpose GPA bits as attribute bits, e.g. to expose an
> execute-only permission bit to the guest. To keep the implementation
> simple (relatively speaking), GPA aliasing is only supported via TDP.
>
> Today KVM assumes two things that are broken by GPA aliasing.
> 1. GPAs coming from hardware can be simply shifted to get the GFNs.
> 2. GPA bits 51:MAXPHYADDR are reserved to zero.
>
> With GPA aliasing, translating a GPA to GFN requires masking off the
> repurposed bit, and a repurposed bit may reside in 51:MAXPHYADDR.
>
> To support GPA aliasing, introduce the concept of per-VM GPA stolen bits,
> that is, bits stolen from the GPA to act as new virtualized attribute
> bits. A bit in the mask will cause the MMU code to create aliases of the
> GPA. It can also be used to find the GFN out of a GPA coming from a tdp
> fault.
>
> To handle case (1) from above, retain any stolen bits when passing a GPA
> in KVM's MMU code, but strip them when converting to a GFN so that the
> GFN contains only the "real" GFN, i.e. never has repurposed bits set.
>
> GFNs (without stolen bits) continue to be used to:
> -Specify physical memory by userspace via memslots
> -Map GPAs to TDP PTEs via RMAP
> -Specify dirty tracking and write protection
> -Look up MTRR types
> -Inject async page faults
>
> Since there are now multiple aliases for the same aliased GPA, when
> userspace memory backing the memslots is paged out, both aliases need to be
> modified. Fortunately this happens automatically. Since rmap supports
> multiple mappings for the same GFN for PTE shadowing based paging, by
> adding/removing each alias PTE with its GFN, kvm_handle_hva() based
> operations will be applied to both aliases.
>
> In the case of the rmap being removed in the future, the needed
> information could be recovered by iterating over the stolen bits and
> walking the TDP page tables.
>
> For TLB flushes that are address based, make sure to flush both aliases
> in the stolen bits case.
>
> Only support stolen bits in 64 bit guest paging modes (long, PAE).
> Features that use this infrastructure should restrict the stolen bits to
> exclude the other paging modes. Don't support stolen bits for shadow EPT.
>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> ---
> arch/x86/kvm/mmu.h | 26 ++++++++++
> arch/x86/kvm/mmu/mmu.c | 86 ++++++++++++++++++++++-----------
> arch/x86/kvm/mmu/mmu_internal.h | 1 +
> arch/x86/kvm/mmu/paging_tmpl.h | 25 ++++++----
> 4 files changed, 101 insertions(+), 37 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index 88d0ed5225a4..69b82857acdb 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -232,4 +232,30 @@ int kvm_arch_write_log_dirty(struct kvm_vcpu *vcpu);
> int kvm_mmu_post_init_vm(struct kvm *kvm);
> void kvm_mmu_pre_destroy_vm(struct kvm *kvm);
>
> +static inline gfn_t kvm_gfn_stolen_mask(struct kvm *kvm)
> +{
> + /* Currently there are no stolen bits in KVM */
> + return 0;
> +}
> +
> +static inline gfn_t vcpu_gfn_stolen_mask(struct kvm_vcpu *vcpu)
> +{
> + return kvm_gfn_stolen_mask(vcpu->kvm);
> +}
> +
> +static inline gpa_t kvm_gpa_stolen_mask(struct kvm *kvm)
> +{
> + return kvm_gfn_stolen_mask(kvm) << PAGE_SHIFT;
> +}
> +
> +static inline gpa_t vcpu_gpa_stolen_mask(struct kvm_vcpu *vcpu)
> +{
> + return kvm_gpa_stolen_mask(vcpu->kvm);
> +}
> +
> +static inline gfn_t vcpu_gpa_to_gfn_unalias(struct kvm_vcpu *vcpu, gpa_t gpa)
> +{
> + return (gpa >> PAGE_SHIFT) & ~vcpu_gfn_stolen_mask(vcpu);
> +}
> +
> #endif
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 0dc4bf34ce9c..990ee645b8a2 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -188,27 +188,37 @@ static inline bool kvm_available_flush_tlb_with_range(void)
> return kvm_x86_ops.tlb_remote_flush_with_range;
> }
>
> -static void kvm_flush_remote_tlbs_with_range(struct kvm *kvm,
> - struct kvm_tlb_range *range)
> -{
> - int ret = -ENOTSUPP;
> -
> - if (range && kvm_x86_ops.tlb_remote_flush_with_range)
> - ret = static_call(kvm_x86_tlb_remote_flush_with_range)(kvm, range);
> -
> - if (ret)
> - kvm_flush_remote_tlbs(kvm);
> -}
> -
> void kvm_flush_remote_tlbs_with_address(struct kvm *kvm,
> u64 start_gfn, u64 pages)
> {
> struct kvm_tlb_range range;
> + u64 gfn_stolen_mask;
> +
> + if (!kvm_available_flush_tlb_with_range())
> + goto generic_flush;
> +
> + /*
> + * Fall back to the big hammer flush if there is more than one
> + * GPA alias that needs to be flushed.
> + */
> + gfn_stolen_mask = kvm_gfn_stolen_mask(kvm);
> + if (hweight64(gfn_stolen_mask) > 1)
> + goto generic_flush;
>
> range.start_gfn = start_gfn;
> range.pages = pages;
> + if (static_call(kvm_x86_tlb_remote_flush_with_range)(kvm, &range))
> + goto generic_flush;
> +
> + if (!gfn_stolen_mask)
> + return;
>
> - kvm_flush_remote_tlbs_with_range(kvm, &range);
> + range.start_gfn |= gfn_stolen_mask;
> + static_call(kvm_x86_tlb_remote_flush_with_range)(kvm, &range);
> + return;
> +
> +generic_flush:
> + kvm_flush_remote_tlbs(kvm);
> }
>
> bool is_nx_huge_page_enabled(void)
> @@ -1949,14 +1959,16 @@ static void clear_sp_write_flooding_count(u64 *spte)
> __clear_sp_write_flooding_count(sptep_to_sp(spte));
> }
>
> -static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
> - gfn_t gfn,
> - gva_t gaddr,
> - unsigned level,
> - int direct,
> - unsigned int access)
> +static struct kvm_mmu_page *__kvm_mmu_get_page(struct kvm_vcpu *vcpu,
> + gfn_t gfn,
> + gfn_t gfn_stolen_bits,
> + gva_t gaddr,
> + unsigned int level,
> + int direct,
> + unsigned int access)
> {
> bool direct_mmu = vcpu->arch.mmu->direct_map;
> + gpa_t gfn_and_stolen = gfn | gfn_stolen_bits;
> union kvm_mmu_page_role role;
> struct hlist_head *sp_list;
> unsigned quadrant;
> @@ -1978,9 +1990,9 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
> role.quadrant = quadrant;
> }
>
> - sp_list = &vcpu->kvm->arch.mmu_page_hash[kvm_page_table_hashfn(gfn)];
> + sp_list = &vcpu->kvm->arch.mmu_page_hash[kvm_page_table_hashfn(gfn_and_stolen)];
> for_each_valid_sp(vcpu->kvm, sp, sp_list) {
> - if (sp->gfn != gfn) {
> + if ((sp->gfn | sp->gfn_stolen_bits) != gfn_and_stolen) {
> collisions++;
> continue;
> }
> @@ -2020,6 +2032,7 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
> sp = kvm_mmu_alloc_page(vcpu, direct);
>
> sp->gfn = gfn;
> + sp->gfn_stolen_bits = gfn_stolen_bits;
> sp->role = role;
> hlist_add_head(&sp->hash_link, sp_list);
> if (!direct) {
> @@ -2044,6 +2057,13 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
> return sp;
> }

Sorry for replying old thread, but to me it looks weird to have gfn_stolen_bits
in 'struct kvm_mmu_page'. If I understand correctly, above code basically
means that GFN with different stolen bit will have different 'struct
kvm_mmu_page', but in the context of this patch, mappings with different
stolen bits still use the same root, which means gfn_stolen_bits doesn't make a
lot of sense at least for root page table.

Instead, having gfn_stolen_bits in 'struct kvm_mmu_page' only makes sense in
the context of TDX, since TDX requires two separate roots for private and
shared mappings.

So given we cannot tell whether the same root, or different roots should be
used for different stolen bits, I think we should not add 'gfn_stolen_bits' to
'struct kvm_mmu_page' and use it to determine whether to allocate a new table
for the same GFN, but should use a new role (i.e role.private) to determine.

And removing 'gfn_stolen_bits' in 'struct kvm_mmu_page' could also save some
memory.

>
> +static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu, gfn_t gfn,
> + gva_t gaddr, unsigned int level,
> + int direct, unsigned int access)
> +{
> + return __kvm_mmu_get_page(vcpu, gfn, 0, gaddr, level, direct, access);
> +}
> +
> static void shadow_walk_init_using_root(struct kvm_shadow_walk_iterator *iterator,
> struct kvm_vcpu *vcpu, hpa_t root,
> u64 addr)
> @@ -2637,7 +2657,9 @@ static int direct_pte_prefetch_many(struct kvm_vcpu *vcpu,
>
> gfn = kvm_mmu_page_get_gfn(sp, start - sp->spt);
> slot = gfn_to_memslot_dirty_bitmap(vcpu, gfn, access & ACC_WRITE_MASK);
> - if (!slot)
> +
> + /* Don't map private memslots for stolen bits */
> + if (!slot || (sp->gfn_stolen_bits && slot->id >= KVM_USER_MEM_SLOTS))
> return -1;
>
> ret = gfn_to_page_many_atomic(slot, gfn, pages, end - start);
> @@ -2827,7 +2849,9 @@ static int __direct_map(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
> struct kvm_shadow_walk_iterator it;
> struct kvm_mmu_page *sp;
> int level, req_level, ret;
> - gfn_t gfn = gpa >> PAGE_SHIFT;
> + gpa_t gpa_stolen_mask = vcpu_gpa_stolen_mask(vcpu);
> + gfn_t gfn = (gpa & ~gpa_stolen_mask) >> PAGE_SHIFT;
> + gfn_t gfn_stolen_bits = (gpa & gpa_stolen_mask) >> PAGE_SHIFT;
> gfn_t base_gfn = gfn;
>
> if (WARN_ON(!VALID_PAGE(vcpu->arch.mmu->root_hpa)))
> @@ -2852,8 +2876,9 @@ static int __direct_map(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
>
> drop_large_spte(vcpu, it.sptep);
> if (!is_shadow_present_pte(*it.sptep)) {
> - sp = kvm_mmu_get_page(vcpu, base_gfn, it.addr,
> - it.level - 1, true, ACC_ALL);
> + sp = __kvm_mmu_get_page(vcpu, base_gfn,
> + gfn_stolen_bits, it.addr,
> + it.level - 1, true, ACC_ALL);
>
> link_shadow_page(vcpu, it.sptep, sp);
> if (is_tdp && huge_page_disallowed &&
> @@ -3689,6 +3714,13 @@ static bool try_async_pf(struct kvm_vcpu *vcpu, bool prefault, gfn_t gfn,
> if (slot && (slot->flags & KVM_MEMSLOT_INVALID))
> return true;
>
> + /* Don't expose aliases for no slot GFNs or private memslots */
> + if ((cr2_or_gpa & vcpu_gpa_stolen_mask(vcpu)) &&
> + !kvm_is_visible_memslot(slot)) {
> + *pfn = KVM_PFN_NOSLOT;
> + return false;
> + }
> +
> /* Don't expose private memslots to L2. */
> if (is_guest_mode(vcpu) && !kvm_is_visible_memslot(slot)) {
> *pfn = KVM_PFN_NOSLOT;
> @@ -3723,7 +3755,7 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
> bool write = error_code & PFERR_WRITE_MASK;
> bool map_writable;
>
> - gfn_t gfn = gpa >> PAGE_SHIFT;
> + gfn_t gfn = vcpu_gpa_to_gfn_unalias(vcpu, gpa);
> unsigned long mmu_seq;
> kvm_pfn_t pfn;
> hva_t hva;
> @@ -3833,7 +3865,7 @@ int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
> max_level > PG_LEVEL_4K;
> max_level--) {
> int page_num = KVM_PAGES_PER_HPAGE(max_level);
> - gfn_t base = (gpa >> PAGE_SHIFT) & ~(page_num - 1);
> + gfn_t base = vcpu_gpa_to_gfn_unalias(vcpu, gpa) & ~(page_num - 1);
>
> if (kvm_mtrr_check_gfn_range_consistency(vcpu, base, page_num))
> break;
> diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> index d64ccb417c60..c896ec9f3159 100644
> --- a/arch/x86/kvm/mmu/mmu_internal.h
> +++ b/arch/x86/kvm/mmu/mmu_internal.h
> @@ -46,6 +46,7 @@ struct kvm_mmu_page {
> */
> union kvm_mmu_page_role role;
> gfn_t gfn;
> + gfn_t gfn_stolen_bits;
>
> u64 *spt;
> /* hold the gfn of each spte inside spt */
> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index 823a5919f9fa..439dc141391b 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -25,7 +25,8 @@
> #define guest_walker guest_walker64
> #define FNAME(name) paging##64_##name
> #define PT_BASE_ADDR_MASK PT64_BASE_ADDR_MASK
> - #define PT_LVL_ADDR_MASK(lvl) PT64_LVL_ADDR_MASK(lvl)
> + #define PT_LVL_ADDR_MASK(vcpu, lvl) (~vcpu_gpa_stolen_mask(vcpu) & \
> + PT64_LVL_ADDR_MASK(lvl))
> #define PT_LVL_OFFSET_MASK(lvl) PT64_LVL_OFFSET_MASK(lvl)
> #define PT_INDEX(addr, level) PT64_INDEX(addr, level)
> #define PT_LEVEL_BITS PT64_LEVEL_BITS
> @@ -44,7 +45,7 @@
> #define guest_walker guest_walker32
> #define FNAME(name) paging##32_##name
> #define PT_BASE_ADDR_MASK PT32_BASE_ADDR_MASK
> - #define PT_LVL_ADDR_MASK(lvl) PT32_LVL_ADDR_MASK(lvl)
> + #define PT_LVL_ADDR_MASK(vcpu, lvl) PT32_LVL_ADDR_MASK(lvl)
> #define PT_LVL_OFFSET_MASK(lvl) PT32_LVL_OFFSET_MASK(lvl)
> #define PT_INDEX(addr, level) PT32_INDEX(addr, level)
> #define PT_LEVEL_BITS PT32_LEVEL_BITS
> @@ -58,7 +59,7 @@
> #define guest_walker guest_walkerEPT
> #define FNAME(name) ept_##name
> #define PT_BASE_ADDR_MASK PT64_BASE_ADDR_MASK
> - #define PT_LVL_ADDR_MASK(lvl) PT64_LVL_ADDR_MASK(lvl)
> + #define PT_LVL_ADDR_MASK(vcpu, lvl) PT64_LVL_ADDR_MASK(lvl)
> #define PT_LVL_OFFSET_MASK(lvl) PT64_LVL_OFFSET_MASK(lvl)
> #define PT_INDEX(addr, level) PT64_INDEX(addr, level)
> #define PT_LEVEL_BITS PT64_LEVEL_BITS
> @@ -75,7 +76,7 @@
> #define PT_GUEST_ACCESSED_MASK (1 << PT_GUEST_ACCESSED_SHIFT)
>
> #define gpte_to_gfn_lvl FNAME(gpte_to_gfn_lvl)
> -#define gpte_to_gfn(pte) gpte_to_gfn_lvl((pte), PG_LEVEL_4K)
> +#define gpte_to_gfn(vcpu, pte) gpte_to_gfn_lvl(vcpu, pte, PG_LEVEL_4K)
>
> /*
> * The guest_walker structure emulates the behavior of the hardware page
> @@ -96,9 +97,9 @@ struct guest_walker {
> struct x86_exception fault;
> };
>
> -static gfn_t gpte_to_gfn_lvl(pt_element_t gpte, int lvl)
> +static gfn_t gpte_to_gfn_lvl(struct kvm_vcpu *vcpu, pt_element_t gpte, int lvl)
> {
> - return (gpte & PT_LVL_ADDR_MASK(lvl)) >> PAGE_SHIFT;
> + return (gpte & PT_LVL_ADDR_MASK(vcpu, lvl)) >> PAGE_SHIFT;
> }
>
> static inline void FNAME(protect_clean_gpte)(struct kvm_mmu *mmu, unsigned *access,
> @@ -366,7 +367,7 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
> --walker->level;
>
> index = PT_INDEX(addr, walker->level);
> - table_gfn = gpte_to_gfn(pte);
> + table_gfn = gpte_to_gfn(vcpu, pte);
> offset = index * sizeof(pt_element_t);
> pte_gpa = gfn_to_gpa(table_gfn) + offset;
>
> @@ -432,7 +433,7 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
> if (unlikely(errcode))
> goto error;
>
> - gfn = gpte_to_gfn_lvl(pte, walker->level);
> + gfn = gpte_to_gfn_lvl(vcpu, pte, walker->level);
> gfn += (addr & PT_LVL_OFFSET_MASK(walker->level)) >> PAGE_SHIFT;
>
> if (PTTYPE == 32 && walker->level > PG_LEVEL_4K && is_cpuid_PSE36())
> @@ -537,12 +538,14 @@ FNAME(prefetch_gpte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
> gfn_t gfn;
> kvm_pfn_t pfn;
>
> + WARN_ON(gpte & vcpu_gpa_stolen_mask(vcpu));
> +
> if (FNAME(prefetch_invalid_gpte)(vcpu, sp, spte, gpte))
> return false;
>
> pgprintk("%s: gpte %llx spte %p\n", __func__, (u64)gpte, spte);
>
> - gfn = gpte_to_gfn(gpte);
> + gfn = gpte_to_gfn(vcpu, gpte);
> pte_access = sp->role.access & FNAME(gpte_access)(gpte);
> FNAME(protect_clean_gpte)(vcpu->arch.mmu, &pte_access, gpte);
> pfn = pte_prefetch_gfn_to_pfn(vcpu, gfn,
> @@ -652,6 +655,8 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, gpa_t addr,
>
> direct_access = gw->pte_access;
>
> + WARN_ON(addr & vcpu_gpa_stolen_mask(vcpu));
> +
> top_level = vcpu->arch.mmu->root_level;
> if (top_level == PT32E_ROOT_LEVEL)
> top_level = PT32_ROOT_LEVEL;
> @@ -1067,7 +1072,7 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
> continue;
> }
>
> - gfn = gpte_to_gfn(gpte);
> + gfn = gpte_to_gfn(vcpu, gpte);
> pte_access = sp->role.access;
> pte_access &= FNAME(gpte_access)(gpte);
> FNAME(protect_clean_gpte)(vcpu->arch.mmu, &pte_access, gpte);
> --
> 2.25.1
>

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