Re: [PATCH v7 036/102] KVM: x86/mmu: Allow non-zero value for non-present SPTE

Kai Huang
2022-06-30T11:04:04Z

On Mon, 2022-06-27 at 14:53 -0700, isaku.yamahata@intel.com wrote:
> From: Sean Christopherson <sean.j.christopherson@intel.com>
>
> TDX introduced a new ETP, Secure-EPT, in addition to the existing EPT.
> Secure-EPT maps protected guest memory, which is called private. Since
> Secure-EPT page tables is also protected, those page tables is also called
> private. The existing EPT is often called shared EPT to distinguish from
> Secure-EPT. And also page tables for share EPT is also called shared.

Does this patch has anything to do with secure-EPT?

>
> Virtualization Exception, #VE, is a new processor exception in VMX non-root

#VE isn't new. It's already in pre-TDX public spec AFAICT.

> operation. In certain virtualizatoin-related conditions, #VE is injected
> into guest instead of exiting from guest to VMM so that guest is given a
> chance to inspect it. One important one is EPT violation. When
> "ETP-violation #VE" VM-execution is set, "#VE suppress bit" in EPT entry
> is cleared, #VE is injected instead of EPT violation.

We already know such fact based on pre-TDX public spec. Instead of repeating it
here, why not focusing on saying what's new in TDX, so your below paragraph of
setting a non-zero value for non-present SPTE can be justified?

>
> Because guest memory is protected with TDX, VMM can't parse instructions
> in the guest memory. Instead, MMIO hypercall is used for guest to pass
> necessary information to VMM.
>
> To make unmodified device driver work, guest TD expects #VE on accessing
> shared GPA. The #VE handler converts MMIO access into MMIO hypercall with
> the EPT entry of enabled "#VE" by clearing "suppress #VE" bit. Before VMM
> enabling #VE, it needs to figure out the given GPA is for MMIO by EPT
> violation.
>

As I said above, before here, you need to explain in TDX VMCS is controlled by
the TDX module and it always sets the "EPT-violation #VE" in execution control
bit.

> So the execution flow looks like
>
> - Allocate unused shared EPT entry with suppress #VE bit set.
> - EPT violation on that GPA.
> - VMM figures out the faulted GPA is for MMIO.
> - VMM clears the suppress #VE bit.
> - Guest TD gets #VE, and converts MMIO access into MMIO hypercall.
> - If the GPA maps guest memory, VMM resolves it with guest pages.
>
> For both cases, SPTE needs suppress #VE" bit set initially when it
> is allocated or zapped, therefore non-zero non-present value for SPTE
> needs to be allowed.
>
> This change requires to update FNAME(sync_page) for shadow EPT.
> "if(!sp->spte[i])" in FNAME(sync_page) means that the spte entry is the
> initial value. With the introduction of shadow_nonpresent_value which can
> be non-zero, it doesn't hold any more. Replace zero check with
> "!is_shadow_present_pte() && !is_mmio_spte()".

I don't think you need to mention above paragraph. It's absolutely unclear how
is_mmio_spte() will be impacted by this patch by reading above paragraphs.

From the "execution flow" you mentioned above, you will change MMIO fault from
EPT misconfiguration to EPT violation (in order to get #VE), so theoretically
you may effectively disable MMIO caching, in which case, if I understand
correctly, is_mmio_spte() always returns false.

I guess you can just change to check:

if (sp->spte[i] != shadow_nonpresent_value)

Anyway, IMO you can just comment in the code.

After all, what is shadow_nonpresent_value, given you haven't explained what it
is?

>
> When "if (!spt[i])" doesn't hold, but the entry value is
> shadow_nonpresent_value, the entry is wrongly synchronized from non-present
> to non-present with (wrongly) pfn changed and tries to remove rmap wrongly
> and BUG_ON() is hit.

Ditto.

>
> TDP MMU uses REMOVED_SPTE = 0x5a0ULL as special constant to indicate the
> intermediate value to indicate one thread is operating on it and the value
> should be semi-arbitrary value. For TDX (more correctly to use #VE), the
> value should include suppress #VE value which is SHADOW_NONPRESENT_VALUE.

What is SHADOW_NONPRESENT_VALUE?

> Rename REMOVED_SPTE to __REMOVED_SPTE and define REMOVED_SPTE as
> SHADOW_NONPRESENT_VALUE | REMOVED_SPTE to set suppress #VE bit.

Ditto. IMHO you don't even need to mention REMOVED_SPTE in changelog.

>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> ---
> arch/x86/kvm/mmu/mmu.c | 55 ++++++++++++++++++++++++++++++----
> arch/x86/kvm/mmu/paging_tmpl.h | 3 +-
> arch/x86/kvm/mmu/spte.c | 5 +++-
> arch/x86/kvm/mmu/spte.h | 37 ++++++++++++++++++++---
> arch/x86/kvm/mmu/tdp_mmu.c | 23 +++++++++-----
> 5 files changed, 105 insertions(+), 18 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 51306b80f47c..f239b6cb5d53 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -668,6 +668,44 @@ static void walk_shadow_page_lockless_end(struct kvm_vcpu *vcpu)
> }
> }
>
> +static inline void kvm_init_shadow_page(void *page)
> +{
> +#ifdef CONFIG_X86_64
> + int ign;
> +
> + WARN_ON_ONCE(shadow_nonpresent_value != SHADOW_NONPRESENT_VALUE);
> + asm volatile (
> + "rep stosq\n\t"
> + : "=c"(ign), "=D"(page)
> + : "a"(SHADOW_NONPRESENT_VALUE), "c"(4096/8), "D"(page)
> + : "memory"
> + );
> +#else
> + BUG();
> +#endif
> +}
> +
> +static int mmu_topup_shadow_page_cache(struct kvm_vcpu *vcpu)
> +{
> + struct kvm_mmu_memory_cache *mc = &vcpu->arch.mmu_shadow_page_cache;
> + int start, end, i, r;
> + bool is_tdp_mmu = is_tdp_mmu_enabled(vcpu->kvm);
> +
> + if (is_tdp_mmu && shadow_nonpresent_value)
> + start = kvm_mmu_memory_cache_nr_free_objects(mc);
> +
> + r = kvm_mmu_topup_memory_cache(mc, PT64_ROOT_MAX_LEVEL);
> + if (r)
> + return r;
> +
> + if (is_tdp_mmu && shadow_nonpresent_value) {
> + end = kvm_mmu_memory_cache_nr_free_objects(mc);
> + for (i = start; i < end; i++)
> + kvm_init_shadow_page(mc->objects[i]);
> + }

I think you can just extend this to legacy MMU too, but not only TDP MMU.

After all, before this patch, where have you declared that TDX only supports TDP
MMU? This is only enforced in:

[PATCH v7 043/102] KVM: x86/mmu: Focibly use TDP MMU for TDX

Which is 7 patches later.

Also, shadow_nonpresent_value is only used in couple of places, while
SHADOW_NONPRESENT_VALUE is used directly in more places. Does it make more
sense to always use shadow_nonpresent_value, instead of using
SHADOW_NONPRESENT_VALUE?

Similar to other shadow values, we can provide a function to let caller
(VMX/SVM) to decide whether it wants to use non-zero for non-present SPTE.

void kvm_mmu_set_non_present_value(u64 value)
{
shadow_nonpresent_value = value;
}

> + return 0;
> +}
> +
> static int mmu_topup_memory_caches(struct kvm_vcpu *vcpu, bool maybe_indirect)
> {
> int r;
> @@ -677,8 +715,7 @@ static int mmu_topup_memory_caches(struct kvm_vcpu *vcpu, bool maybe_indirect)
> 1 + PT64_ROOT_MAX_LEVEL + PTE_PREFETCH_NUM);
> if (r)
> return r;
> - r = kvm_mmu_topup_memory_cache(&vcpu->arch.mmu_shadow_page_cache,
> - PT64_ROOT_MAX_LEVEL);
> + r = mmu_topup_shadow_page_cache(vcpu);
> if (r)
> return r;
> if (maybe_indirect) {
> @@ -5521,9 +5558,16 @@ void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level,
> * what is used by the kernel for any given HVA, i.e. the kernel's
> * capabilities are ultimately consulted by kvm_mmu_hugepage_adjust().
> */
> - if (tdp_enabled)
> + if (tdp_enabled) {
> + /*
> + * For TDP MMU, always set bit 63 for TDX support. See the
> + * comment on SHADOW_NONPRESENT_VALUE.
> + */
> +#ifdef CONFIG_X86_64
> + shadow_nonpresent_value = SHADOW_NONPRESENT_VALUE;
> +#endif

'tdp_enabled' doesn't mean TDP MMU, right?

> max_huge_page_level = tdp_huge_page_level;
> - else if (boot_cpu_has(X86_FEATURE_GBPAGES))
> + } else if (boot_cpu_has(X86_FEATURE_GBPAGES))
> max_huge_page_level = PG_LEVEL_1G;
> else
> max_huge_page_level = PG_LEVEL_2M;
> @@ -5654,7 +5698,8 @@ int kvm_mmu_create(struct kvm_vcpu *vcpu)
> vcpu->arch.mmu_page_header_cache.kmem_cache = mmu_page_header_cache;
> vcpu->arch.mmu_page_header_cache.gfp_zero = __GFP_ZERO;
>
> - vcpu->arch.mmu_shadow_page_cache.gfp_zero = __GFP_ZERO;
> + if (!(is_tdp_mmu_enabled(vcpu->kvm) && shadow_nonpresent_value))
> + vcpu->arch.mmu_shadow_page_cache.gfp_zero = __GFP_ZERO;
>
> vcpu->arch.mmu = &vcpu->arch.root_mmu;
> vcpu->arch.walk_mmu = &vcpu->arch.root_mmu;
> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index fe35d8fd3276..ee2fb0c073f3 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -1031,7 +1031,8 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
> gpa_t pte_gpa;
> gfn_t gfn;
>
> - if (!sp->spt[i])
> + if (!is_shadow_present_pte(sp->spt[i]) &&
> + !is_mmio_spte(sp->spt[i]))
> continue;

As I said in the changelog, I don't think this is correct.

I guess you can just change to check:

if (sp->spte[i] != shadow_nonpresent_value)

>
> pte_gpa = first_pte_gpa + i * sizeof(pt_element_t);
> diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
> index cda1851ec155..bd441458153f 100644
> --- a/arch/x86/kvm/mmu/spte.c
> +++ b/arch/x86/kvm/mmu/spte.c
> @@ -36,6 +36,9 @@ u64 __read_mostly shadow_present_mask;
> u64 __read_mostly shadow_me_value;
> u64 __read_mostly shadow_me_mask;
> u64 __read_mostly shadow_acc_track_mask;
> +#ifdef CONFIG_X86_64
> +u64 __read_mostly shadow_nonpresent_value;
> +#endif
>
> u64 __read_mostly shadow_nonpresent_or_rsvd_mask;
> u64 __read_mostly shadow_nonpresent_or_rsvd_lower_gfn_mask;
> @@ -360,7 +363,7 @@ void kvm_mmu_set_mmio_spte_mask(u64 mmio_value, u64 mmio_mask, u64 access_mask)
> * not set any RWX bits.
> */
> if (WARN_ON((mmio_value & mmio_mask) != mmio_value) ||
> - WARN_ON(mmio_value && (REMOVED_SPTE & mmio_mask) == mmio_value))
> + WARN_ON(mmio_value && (__REMOVED_SPTE & mmio_mask) == mmio_value))
> mmio_value = 0;
>
> if (!mmio_value)
> diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> index 0127bb6e3c7d..1bfedbe0585f 100644
> --- a/arch/x86/kvm/mmu/spte.h
> +++ b/arch/x86/kvm/mmu/spte.h
> @@ -140,6 +140,19 @@ static_assert(MMIO_SPTE_GEN_LOW_BITS == 8 && MMIO_SPTE_GEN_HIGH_BITS == 11);
>
> #define MMIO_SPTE_GEN_MASK GENMASK_ULL(MMIO_SPTE_GEN_LOW_BITS + MMIO_SPTE_GEN_HIGH_BITS - 1, 0)
>
> +/*
> + * non-present SPTE value for both VMX and SVM for TDP MMU.
> + * For SVM NPT, for non-present spte (bit 0 = 0), other bits are ignored.
> + * For VMX EPT, bit 63 is ignored if #VE is disabled.
> + * bit 63 is #VE suppress if #VE is enabled.
> + */
> +#ifdef CONFIG_X86_64
> +#define SHADOW_NONPRESENT_VALUE BIT_ULL(63)
> +static_assert(!(SHADOW_NONPRESENT_VALUE & SPTE_MMU_PRESENT_MASK));
> +#else
> +#define SHADOW_NONPRESENT_VALUE 0ULL
> +#endif
> +
> extern u64 __read_mostly shadow_host_writable_mask;
> extern u64 __read_mostly shadow_mmu_writable_mask;
> extern u64 __read_mostly shadow_nx_mask;
> @@ -154,6 +167,12 @@ extern u64 __read_mostly shadow_present_mask;
> extern u64 __read_mostly shadow_me_value;
> extern u64 __read_mostly shadow_me_mask;
>
> +#ifdef CONFIG_X86_64
> +extern u64 __read_mostly shadow_nonpresent_value;
> +#else
> +#define shadow_nonpresent_value 0ULL
> +#endif
> +
> /*
> * SPTEs in MMUs without A/D bits are marked with SPTE_TDP_AD_DISABLED_MASK;
> * shadow_acc_track_mask is the set of bits to be cleared in non-accessed
> @@ -174,9 +193,12 @@ extern u64 __read_mostly shadow_nonpresent_or_rsvd_mask;
>
> /*
> * If a thread running without exclusive control of the MMU lock must perform a
> - * multi-part operation on an SPTE, it can set the SPTE to REMOVED_SPTE as a
> + * multi-part operation on an SPTE, it can set the SPTE to __REMOVED_SPTE as a
> * non-present intermediate value. Other threads which encounter this value
> - * should not modify the SPTE.
> + * should not modify the SPTE. For the case that TDX is enabled,
> + * SHADOW_NONPRESENT_VALUE, which is "suppress #VE" bit set because TDX module
> + * always enables "EPT violation #VE". The bit is ignored by non-TDX case as
> + * present bit (bit 0) is cleared.
> *
> * Use a semi-arbitrary value that doesn't set RWX bits, i.e. is not-present on
> * bot AMD and Intel CPUs, and doesn't set PFN bits, i.e. doesn't create a L1TF
> @@ -184,10 +206,17 @@ extern u64 __read_mostly shadow_nonpresent_or_rsvd_mask;
> *
> * Only used by the TDP MMU.
> */
> -#define REMOVED_SPTE 0x5a0ULL
> +#define __REMOVED_SPTE 0x5a0ULL
>
> /* Removed SPTEs must not be misconstrued as shadow present PTEs. */
> -static_assert(!(REMOVED_SPTE & SPTE_MMU_PRESENT_MASK));
> +static_assert(!(__REMOVED_SPTE & SPTE_MMU_PRESENT_MASK));
> +static_assert(!(__REMOVED_SPTE & SHADOW_NONPRESENT_VALUE));

I don't think you need this. My understanding of the reason REMOVED_SPTE is
checked against SPTE_MMU_PRESENT_MASK is because they both at low 12 bits.
SHADOW_NONPRESENT_VALUE is bit 63 so it's not possible to conflict with
REMOVED_SPTE, which by comment only uses low bits.

> +
> +/*
> + * See above comment around __REMOVED_SPTE. REMOVED_SPTE is the actual
> + * intermediate value set to the removed SPET. it sets the "suppress #VE" bit.
> + */
> +#define REMOVED_SPTE (SHADOW_NONPRESENT_VALUE | __REMOVED_SPTE)
>
> static inline bool is_removed_spte(u64 spte)
> {
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 7b9265d67131..2ca03ec3bf52 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -692,8 +692,16 @@ static inline int tdp_mmu_zap_spte_atomic(struct kvm *kvm,
> * overwrite the special removed SPTE value. No bookkeeping is needed
> * here since the SPTE is going from non-present to non-present. Use
> * the raw write helper to avoid an unnecessary check on volatile bits.
> + *
> + * Set non-present value to SHADOW_NONPRESENT_VALUE, rather than 0.
> + * It is because when TDX is enabled, TDX module always
> + * enables "EPT-violation #VE", so KVM needs to set
> + * "suppress #VE" bit in EPT table entries, in order to get
> + * real EPT violation, rather than TDVMCALL. KVM sets
> + * SHADOW_NONPRESENT_VALUE (which sets "suppress #VE" bit) so it
> + * can be set when EPT table entries are zapped.
> */
> - __kvm_tdp_mmu_write_spte(iter->sptep, 0);
> + __kvm_tdp_mmu_write_spte(iter->sptep, SHADOW_NONPRESENT_VALUE);
>
> return 0;
> }
> @@ -870,8 +878,8 @@ static void __tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
> continue;
>
> if (!shared)
> - tdp_mmu_set_spte(kvm, &iter, 0);
> - else if (tdp_mmu_set_spte_atomic(kvm, &iter, 0))
> + tdp_mmu_set_spte(kvm, &iter, SHADOW_NONPRESENT_VALUE);
> + else if (tdp_mmu_set_spte_atomic(kvm, &iter, SHADOW_NONPRESENT_VALUE))
> goto retry;
> }
> }
> @@ -927,8 +935,9 @@ bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
> if (WARN_ON_ONCE(!is_shadow_present_pte(old_spte)))
> return false;
>
> - __tdp_mmu_set_spte(kvm, kvm_mmu_page_as_id(sp), sp->ptep, old_spte, 0,
> - sp->gfn, sp->role.level + 1, true, true);
> + __tdp_mmu_set_spte(kvm, kvm_mmu_page_as_id(sp), sp->ptep, old_spte,
> + SHADOW_NONPRESENT_VALUE, sp->gfn, sp->role.level + 1,
> + true, true);
>
> return true;
> }
> @@ -965,7 +974,7 @@ static bool tdp_mmu_zap_leafs(struct kvm *kvm, struct kvm_mmu_page *root,
> !is_last_spte(iter.old_spte, iter.level))
> continue;
>
> - tdp_mmu_set_spte(kvm, &iter, 0);
> + tdp_mmu_set_spte(kvm, &iter, SHADOW_NONPRESENT_VALUE);
> flush = true;
> }
>
> @@ -1330,7 +1339,7 @@ static bool set_spte_gfn(struct kvm *kvm, struct tdp_iter *iter,
> * invariant that the PFN of a present * leaf SPTE can never change.
> * See __handle_changed_spte().
> */
> - tdp_mmu_set_spte(kvm, iter, 0);
> + tdp_mmu_set_spte(kvm, iter, SHADOW_NONPRESENT_VALUE);
>
> if (!pte_write(range->pte)) {
> new_spte = kvm_mmu_changed_pte_notifier_make_spte(iter->old_spte,