Re: [PATCH 04/21] KVM: x86/mmu: Allow non-zero value for non-present SPTE and removed SPTE

[Xu Yilun]

On Tue, Feb 27, 2024 at 06:20:43PM -0500, Paolo Bonzini wrote:
> From: Sean Christopherson <seanjc@xxxxxxxxxx>
> 
> 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 make hypercall to KVM to get the expected information.
> 
> To achieve this, the TDX module always enables "EPT-violation #VE" in the
> VMCS control.  And accordingly, for the MMIO spte for the shared GPA,
> 1. KVM needs to set "suppress #VE" bit for the non-present SPTE so that EPT
> violation happens on TD accessing MMIO range.  2. On EPT violation, KVM
> sets the MMIO spte to clear "suppress #VE" bit so the TD guest can receive
> the #VE instead of EPT misconfiguration unlike VMX case.  For the shared GPA
> that is not populated yet, EPT violation need to be triggered when TD guest
> accesses such shared GPA.  The non-present SPTE value for shared GPA should
> set "suppress #VE" bit.
> 
> 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 <seanjc@xxxxxxxxxx>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> Reviewed-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
> Message-Id: <a99cb866897c7083430dce7f24c63b17d7121134.1705965635.git.isaku.yamahata@xxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/spte.h | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> index 4d1799ba2bf8..26bc95bbc962 100644
> --- a/arch/x86/kvm/mmu/spte.h
> +++ b/arch/x86/kvm/mmu/spte.h
> @@ -149,7 +149,20 @@ 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. (EPT_VIOLATION_VE=0)
> + *              bit 63 is #VE suppress if #VE is enabled. (EPT_VIOLATION_VE=1)
> + * For TDX:
> + *   TDX module sets EPT_VIOLATION_VE for Secure-EPT and conventional EPT
> + */
> +#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;
> @@ -196,7 +209,7 @@ extern u64 __read_mostly shadow_nonpresent_or_rsvd_mask;

    * vulnerability.  Use only low bits to avoid 64-bit immediates.
                      ^

We may remove this comment. Others are fine.

Reviewed-by: Xu Yilun <yilun.xu@xxxxxxxxxxxxxxx>

>   *
>   * 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));
> -- 
> 2.39.0
> 
> 
>