linux-kernel - Re: [RFC PATCH 5/9] KVM: x86/mmu: Free up A/D bits in FROZEN

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Message-ID: <20240805072013.i3ib4h7eadlzzglm@yy-desk-7060>
Date: Mon, 5 Aug 2024 15:20:13 +0800
From: Yuan Yao <yuan.yao@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 5/9] KVM: x86/mmu: Free up A/D bits in FROZEN_SPTE

On Thu, Aug 01, 2024 at 11:34:49AM -0700, Sean Christopherson wrote:
> Remove all flavors of A/D bits from FROZEN_SPTE so that KVM can keep A/D
> bits set in SPTEs that are frozen, without getting false positives.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/mmu/spte.h | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> index ba7ff1dfbeb2..d403ecdfcb8e 100644
> --- a/arch/x86/kvm/mmu/spte.h
> +++ b/arch/x86/kvm/mmu/spte.h
> @@ -216,15 +216,17 @@ extern u64 __read_mostly shadow_nonpresent_or_rsvd_mask;
>   * should not modify the SPTE.
>   *
>   * Use a semi-arbitrary value that doesn't set RWX bits, i.e. is not-present on
> - * both AMD and Intel CPUs, and doesn't set PFN bits, i.e. doesn't create a L1TF
> - * vulnerability.
> + * both AMD and Intel CPUs, doesn't set any A/D bits, and doesn't set PFN bits,
> + * i.e. doesn't create a L1TF vulnerability.
>   *
>   * Only used by the TDP MMU.
>   */
> -#define FROZEN_SPTE	(SHADOW_NONPRESENT_VALUE | 0x5a0ULL)
> +#define FROZEN_SPTE	(SHADOW_NONPRESENT_VALUE | 0x498ULL)

Question:
Why bit3 and bit4 also changed from 0 to 1 ?
They're not part of AD bits fro EPT and CR3 page table/AMD NPT

EPT: Abit:8 Dbit:9
CR3: Abit:5 Dbit:6

>
>  /* Removed SPTEs must not be misconstrued as shadow present PTEs. */
>  static_assert(!(FROZEN_SPTE & SPTE_MMU_PRESENT_MASK));
> +static_assert(!(FROZEN_SPTE & (PT_ACCESSED_MASK | VMX_EPT_ACCESS_BIT)));
> +static_assert(!(FROZEN_SPTE & (PT_DIRTY_MASK | VMX_EPT_DIRTY_BIT)));
>
>  static inline bool is_frozen_spte(u64 spte)
>  {
> --
> 2.46.0.rc1.232.g9752f9e123-goog
>
>