Re: [PATCH v9 7/8] KVM: Handle page fault for private memory

From: Isaku Yamahata
Date: Mon Oct 31 2022 - 20:03:04 EST

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On Fri, Oct 28, 2022 at 02:55:45PM +0800,
Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> wrote:

> On Wed, Oct 26, 2022 at 02:54:25PM -0700, Isaku Yamahata wrote:
> > On Tue, Oct 25, 2022 at 11:13:43PM +0800,
> > Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> wrote:
> >
> > > A memslot with KVM_MEM_PRIVATE being set can include both fd-based
> > > private memory and hva-based shared memory. Architecture code (like TDX
> > > code) can tell whether the on-going fault is private or not. This patch
> > > adds a 'is_private' field to kvm_page_fault to indicate this and
> > > architecture code is expected to set it.
> > >
> > > To handle page fault for such memslot, the handling logic is different
> > > depending on whether the fault is private or shared. KVM checks if
> > > 'is_private' matches the host's view of the page (maintained in
> > > mem_attr_array).
> > > - For a successful match, private pfn is obtained with
> > > restrictedmem_get_page () from private fd and shared pfn is obtained
> > > with existing get_user_pages().
> > > - For a failed match, KVM causes a KVM_EXIT_MEMORY_FAULT exit to
> > > userspace. Userspace then can convert memory between private/shared
> > > in host's view and retry the fault.
> > >
> > > Co-developed-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> > > Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> > > Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> > > ---
> > > arch/x86/kvm/mmu/mmu.c | 56 +++++++++++++++++++++++++++++++--
> > > arch/x86/kvm/mmu/mmu_internal.h | 14 ++++++++-
> > > arch/x86/kvm/mmu/mmutrace.h | 1 +
> > > arch/x86/kvm/mmu/spte.h | 6 ++++
> > > arch/x86/kvm/mmu/tdp_mmu.c | 3 +-
> > > include/linux/kvm_host.h | 28 +++++++++++++++++
> > > 6 files changed, 103 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index 67a9823a8c35..10017a9f26ee 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -3030,7 +3030,7 @@ static int host_pfn_mapping_level(struct kvm *kvm, gfn_t gfn,
> > >
> > > int kvm_mmu_max_mapping_level(struct kvm *kvm,
> > > const struct kvm_memory_slot *slot, gfn_t gfn,
> > > - int max_level)
> > > + int max_level, bool is_private)
> > > {
> > > struct kvm_lpage_info *linfo;
> > > int host_level;
> > > @@ -3042,6 +3042,9 @@ int kvm_mmu_max_mapping_level(struct kvm *kvm,
> > > break;
> > > }
> > >
> > > + if (is_private)
> > > + return max_level;
> >
> > Below PG_LEVEL_NUM is passed by zap_collapsible_spte_range(). It doesn't make
> > sense.
> >
> > > +
> > > if (max_level == PG_LEVEL_4K)
> > > return PG_LEVEL_4K;
> > >
> > > @@ -3070,7 +3073,8 @@ void kvm_mmu_hugepage_adjust(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
> > > * level, which will be used to do precise, accurate accounting.
> > > */
> > > fault->req_level = kvm_mmu_max_mapping_level(vcpu->kvm, slot,
> > > - fault->gfn, fault->max_level);
> > > + fault->gfn, fault->max_level,
> > > + fault->is_private);
> > > if (fault->req_level == PG_LEVEL_4K || fault->huge_page_disallowed)
> > > return;
> > >
> > > @@ -4141,6 +4145,32 @@ void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu, struct kvm_async_pf *work)
> > > kvm_mmu_do_page_fault(vcpu, work->cr2_or_gpa, 0, true);
> > > }
> > >
> > > +static inline u8 order_to_level(int order)
> > > +{
> > > + BUILD_BUG_ON(KVM_MAX_HUGEPAGE_LEVEL > PG_LEVEL_1G);
> > > +
> > > + if (order >= KVM_HPAGE_GFN_SHIFT(PG_LEVEL_1G))
> > > + return PG_LEVEL_1G;
> > > +
> > > + if (order >= KVM_HPAGE_GFN_SHIFT(PG_LEVEL_2M))
> > > + return PG_LEVEL_2M;
> > > +
> > > + return PG_LEVEL_4K;
> > > +}
> > > +
> > > +static int kvm_faultin_pfn_private(struct kvm_page_fault *fault)
> > > +{
> > > + int order;
> > > + struct kvm_memory_slot *slot = fault->slot;
> > > +
> > > + if (kvm_restricted_mem_get_pfn(slot, fault->gfn, &fault->pfn, &order))
> > > + return RET_PF_RETRY;
> > > +
> > > + fault->max_level = min(order_to_level(order), fault->max_level);
> > > + fault->map_writable = !(slot->flags & KVM_MEM_READONLY);
> > > + return RET_PF_CONTINUE;
> > > +}
> > > +
> > > static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> > > {
> > > struct kvm_memory_slot *slot = fault->slot;
> > > @@ -4173,6 +4203,22 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> > > return RET_PF_EMULATE;
> > > }
> > >
> > > + if (kvm_slot_can_be_private(slot) &&
> > > + fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
> > > + vcpu->run->exit_reason = KVM_EXIT_MEMORY_FAULT;
> > > + if (fault->is_private)
> > > + vcpu->run->memory.flags = KVM_MEMORY_EXIT_FLAG_PRIVATE;
> > > + else
> > > + vcpu->run->memory.flags = 0;
> > > + vcpu->run->memory.padding = 0;
> > > + vcpu->run->memory.gpa = fault->gfn << PAGE_SHIFT;
> > > + vcpu->run->memory.size = PAGE_SIZE;
> > > + return RET_PF_USER;
> > > + }
> > > +
> > > + if (fault->is_private)
> > > + return kvm_faultin_pfn_private(fault);
> > > +
> > > async = false;
> > > fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, &async,
> > > fault->write, &fault->map_writable,
> > > @@ -5557,6 +5603,9 @@ int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 err
> > > return -EIO;
> > > }
> > >
> > > + if (r == RET_PF_USER)
> > > + return 0;
> > > +
> > > if (r < 0)
> > > return r;
> > > if (r != RET_PF_EMULATE)
> > > @@ -6408,7 +6457,8 @@ static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm,
> > > */
> > > if (sp->role.direct &&
> > > sp->role.level < kvm_mmu_max_mapping_level(kvm, slot, sp->gfn,
> > > - PG_LEVEL_NUM)) {
> > > + PG_LEVEL_NUM,
> > > + false)) {
> > > kvm_zap_one_rmap_spte(kvm, rmap_head, sptep);
> > >
> > > if (kvm_available_flush_tlb_with_range())
> > > diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> > > index 582def531d4d..5cdff5ca546c 100644
> > > --- a/arch/x86/kvm/mmu/mmu_internal.h
> > > +++ b/arch/x86/kvm/mmu/mmu_internal.h
> > > @@ -188,6 +188,7 @@ struct kvm_page_fault {
> > >
> > > /* Derived from mmu and global state. */
> > > const bool is_tdp;
> > > + const bool is_private;
> > > const bool nx_huge_page_workaround_enabled;
> > >
> > > /*
> > > @@ -236,6 +237,7 @@ int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault);
> > > * RET_PF_RETRY: let CPU fault again on the address.
> > > * RET_PF_EMULATE: mmio page fault, emulate the instruction directly.
> > > * RET_PF_INVALID: the spte is invalid, let the real page fault path update it.
> > > + * RET_PF_USER: need to exit to userspace to handle this fault.
> > > * RET_PF_FIXED: The faulting entry has been fixed.
> > > * RET_PF_SPURIOUS: The faulting entry was already fixed, e.g. by another vCPU.
> > > *
> > > @@ -252,6 +254,7 @@ enum {
> > > RET_PF_RETRY,
> > > RET_PF_EMULATE,
> > > RET_PF_INVALID,
> > > + RET_PF_USER,
> > > RET_PF_FIXED,
> > > RET_PF_SPURIOUS,
> > > };
> > > @@ -309,7 +312,7 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
> > >
> > > int kvm_mmu_max_mapping_level(struct kvm *kvm,
> > > const struct kvm_memory_slot *slot, gfn_t gfn,
> > > - int max_level);
> > > + int max_level, bool is_private);
> > > void kvm_mmu_hugepage_adjust(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault);
> > > void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_level);
> > >
> > > @@ -318,4 +321,13 @@ void *mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc);
> > > void account_huge_nx_page(struct kvm *kvm, struct kvm_mmu_page *sp);
> > > void unaccount_huge_nx_page(struct kvm *kvm, struct kvm_mmu_page *sp);
> > >
> > > +#ifndef CONFIG_HAVE_KVM_RESTRICTED_MEM
> > > +static inline int kvm_restricted_mem_get_pfn(struct kvm_memory_slot *slot,
> > > + gfn_t gfn, kvm_pfn_t *pfn, int *order)
> > > +{
> > > + WARN_ON_ONCE(1);
> > > + return -EOPNOTSUPP;
> > > +}
> > > +#endif /* CONFIG_HAVE_KVM_RESTRICTED_MEM */
> > > +
> > > #endif /* __KVM_X86_MMU_INTERNAL_H */
> > > diff --git a/arch/x86/kvm/mmu/mmutrace.h b/arch/x86/kvm/mmu/mmutrace.h
> > > index ae86820cef69..2d7555381955 100644
> > > --- a/arch/x86/kvm/mmu/mmutrace.h
> > > +++ b/arch/x86/kvm/mmu/mmutrace.h
> > > @@ -58,6 +58,7 @@ TRACE_DEFINE_ENUM(RET_PF_CONTINUE);
> > > TRACE_DEFINE_ENUM(RET_PF_RETRY);
> > > TRACE_DEFINE_ENUM(RET_PF_EMULATE);
> > > TRACE_DEFINE_ENUM(RET_PF_INVALID);
> > > +TRACE_DEFINE_ENUM(RET_PF_USER);
> > > TRACE_DEFINE_ENUM(RET_PF_FIXED);
> > > TRACE_DEFINE_ENUM(RET_PF_SPURIOUS);
> > >
> > > diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> > > index 7670c13ce251..9acdf72537ce 100644
> > > --- a/arch/x86/kvm/mmu/spte.h
> > > +++ b/arch/x86/kvm/mmu/spte.h
> > > @@ -315,6 +315,12 @@ static inline bool is_dirty_spte(u64 spte)
> > > return dirty_mask ? spte & dirty_mask : spte & PT_WRITABLE_MASK;
> > > }
> > >
> > > +static inline bool is_private_spte(u64 spte)
> > > +{
> > > + /* FIXME: Query C-bit/S-bit for SEV/TDX. */
> > > + return false;
> > > +}
> > > +
> >
> > PFN encoded in spte doesn't make sense. In VMM for TDX, private-vs-shared is
> > determined by S-bit of GFN.
>
> My understanding is we will have software bit in the spte, will we? In
> current TDX code I see we have SPTE_SHARED_MASK bit defined.

I'm afraid that you're referring old version. It's not. For TDX, gfn needs
to be checked. Which isn't encoded in spte.

> > > static inline u64 get_rsvd_bits(struct rsvd_bits_validate *rsvd_check, u64 pte,
> > > int level)
> > > {
> > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > > index 672f0432d777..9f97aac90606 100644
> > > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > > @@ -1768,7 +1768,8 @@ static void zap_collapsible_spte_range(struct kvm *kvm,
> > > continue;
> > >
> > > max_mapping_level = kvm_mmu_max_mapping_level(kvm, slot,
> > > - iter.gfn, PG_LEVEL_NUM);
> > > + iter.gfn, PG_LEVEL_NUM,
> > > + is_private_spte(iter.old_spte));
> > > if (max_mapping_level < iter.level)
> > > continue;
> >
> > This is to merge pages into a large page on the next kvm page fault. large page
> > support is not yet supported. Let's skip the private slot until large page
> > support is done.
>
> So what your suggestion is passing in a 'false' at this time for
> 'is_private'? Unless we will decide not use the above is_private_spte,
> this code does not hurt, right? is_private_spte() return false before
> we finally get chance to add the large page support.

Let's pass false always for now.
--
Isaku Yamahata <isaku.yamahata@xxxxxxxxx>

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In reply to: Chao Peng: "Re: [PATCH v9 7/8] KVM: Handle page fault for private memory"
Next in thread: Chao Peng: "[PATCH v9 8/8] KVM: Enable and expose KVM_MEM_PRIVATE"
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