Re: [PATCH v10 033/108] KVM: x86/mmu: Allow non-zero value for non-present SPTE and removed SPTE

Huang, Kai
2022-11-09T11:24:35Z

On Sat, 2022-10-29 at 23:22 -0700, isaku.yamahata@intel.com wrote:
> From: Sean Christopherson <sean.j.christopherson@intel.com>
>
> For TD guest, the current way to emulate MMIO doesn't work any more, as KVM
> is not able to access the private memory of TD guest and do the emulation.
> Instead, TD guest expects to receive #VE when it accesses the MMIO and then
> it can explicitly makes hypercall to KVM to get the expected information.
^
make
>
> To achieve this, the TDX module always enables "EPT-violation #VE" in the
> VMCS control. And accordingly, KVM needs to configure the MMIO spte to
> trigger EPT violation (instead of misconfiguration) and at the same time,
> also clear the "suppress #VE" bit so the TD guest can get the #VE instead
> of causing actual EPT violation to KVM.
>
> In order for KVM to be able to have chance to set up the correct SPTE for
> MMIO for TD guest, the default non-present SPTE must have the "suppress
> guest accesses the MMIO.
>

The above sentence is broken.

> Also, when TD guest accesses the actual shared
> memory, it should continue to trigger EPT violation to the KVM instead of
> receiving the #VE (the TDX module guarantees KVM will receive EPT violation
> for private memory access). This means for the shared memory, the SPTE
> also must have the "suppress #VE" bit set for the non-present SPTE.
>
> Add "suppress VE" bit (bit 63) to SHADOW_NONPRESENT_VALUE and REMOVED_SPTE.
> Unconditionally set the "suppress #VE" bit (which is bit 63) for both AMD
> and Intel as: 1) AMD hardware doesn't use this bit when present bit is off;
> 2) for normal VMX guest, KVM never enables the "EPT-violation #VE" in VMCS
> control and "suppress #VE" bit is ignored by hardware.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> ---
> arch/x86/include/asm/vmx.h | 1 +
> arch/x86/kvm/mmu/spte.c | 4 +++-
> arch/x86/kvm/mmu/spte.h | 22 +++++++++++++++++++++-
> arch/x86/kvm/mmu/tdp_mmu.c | 8 ++++++++
> 4 files changed, 33 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/vmx.h b/arch/x86/include/asm/vmx.h
> index 498dc600bd5c..cdbf12c1a83c 100644
> --- a/arch/x86/include/asm/vmx.h
> +++ b/arch/x86/include/asm/vmx.h
> @@ -511,6 +511,7 @@ enum vmcs_field {
> #define VMX_EPT_IPAT_BIT (1ull << 6)
> #define VMX_EPT_ACCESS_BIT (1ull << 8)
> #define VMX_EPT_DIRTY_BIT (1ull << 9)
> +#define VMX_EPT_SUPPRESS_VE_BIT (1ull << 63)
> #define VMX_EPT_RWX_MASK (VMX_EPT_READABLE_MASK | \
> VMX_EPT_WRITABLE_MASK | \
> VMX_EPT_EXECUTABLE_MASK)
> diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
> index 2e08b2a45361..0b97a045c5f0 100644
> --- a/arch/x86/kvm/mmu/spte.c
> +++ b/arch/x86/kvm/mmu/spte.c
> @@ -419,7 +419,9 @@ void kvm_mmu_set_ept_masks(bool has_ad_bits, bool has_exec_only)
> shadow_dirty_mask = has_ad_bits ? VMX_EPT_DIRTY_BIT : 0ull;
> shadow_nx_mask = 0ull;
> shadow_x_mask = VMX_EPT_EXECUTABLE_MASK;
> - shadow_present_mask = has_exec_only ? 0ull : VMX_EPT_READABLE_MASK;
> + /* VMX_EPT_SUPPRESS_VE_BIT is needed for W or X violation. */
> + shadow_present_mask =
> + (has_exec_only ? 0ull : VMX_EPT_READABLE_MASK) | VMX_EPT_SUPPRESS_VE_BIT;

I think this chunk can be in a separate patch since it doesn't handle fault from
non-present to present, which is claimed by the patch title and changelog.

Or I think you need to describe handling W or X fault part in the changelog.

> /*
> * EPT overrides the host MTRRs, and so KVM must program the desired
> * memtype directly into the SPTEs. Note, this mask is just the mask
> diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> index 42ecaa75da15..7e0f79e8f45b 100644
> --- a/arch/x86/kvm/mmu/spte.h
> +++ b/arch/x86/kvm/mmu/spte.h
> @@ -148,7 +148,22 @@ 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.
^
Non-present

> + * 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. (EPT_VIOLATION_VE=0)
> + * bit 63 is #VE suppress if #VE is enabled. (EPT_VIOLATION_VE=1)
> + * For TDX:
> + * Secure-EPT: TDX module sets EPT_VIOLATION_VE for Secure-EPT

"EPT-violation #VE" is a VMCS control, so it applies to both Secure-EPT and
shared-EPT. Looks "for Secure-EPT" is a little bit confusing.

> + * private EPT: "suppress #VE" bit is ignored. CPU doesn't walk it.

Looks "private EPT" just comes out of blue. I am not sure whether you need to
mention it here.

> + * conventional EPT: "suppress #VE" bit must be set to get EPT violation
> + */
> +#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;
> @@ -189,13 +204,18 @@ extern u64 __read_mostly shadow_nonpresent_or_rsvd_mask;
> * non-present intermediate value. Other threads which encounter this value
> * should not modify the SPTE.
> *
> + * For X86_64 case, SHADOW_NONPRESENT_VALUE, "suppress #VE" bit, is set because
> + * "EPT violation #VE" in the secondary VM execution control may be enabled.
> + * Because TDX module sets "EPT violation #VE" for TD, "suppress #VE" bit for
> + * the conventional EPT needs to be set.
> + *

This comment looks duplicated with the above one. Not sure whether needed.

> * 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
> * vulnerability. Use only low bits to avoid 64-bit immediates.
> *
> * Only used by the TDP MMU.
> */
> -#define REMOVED_SPTE 0x5a0ULL
> +#define REMOVED_SPTE (SHADOW_NONPRESENT_VALUE | 0x5a0ULL)
>
> /* Removed SPTEs must not be misconstrued as shadow present PTEs. */
> static_assert(!(REMOVED_SPTE & SPTE_MMU_PRESENT_MASK));
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 38bc4c2f0f1f..1eee9c159958 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -693,6 +693,14 @@ 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, SHADOW_NONPRESENT_VALUE);
>

Ditto.