linux-kernel - Re: [RFC PATCH 09/18] KVM: x86/mmu: Extend access bitfield in kvm_mmu_page

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Message-ID: <aCI-z5vzzLwxOCfw@google.com>
Date: Mon, 12 May 2025 11:32:47 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Jon Kohler <jon@...anix.com>
Cc: pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	"Mickaël Salaün" <mic@...ikod.net>, Sergey Dyasli <sergey.dyasli@...anix.com>
Subject: Re: [RFC PATCH 09/18] KVM: x86/mmu: Extend access bitfield in kvm_mmu_page_role

On Thu, Mar 13, 2025, Jon Kohler wrote:
> diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> index 71d6fe28fafc..d9e22133b6d0 100644
> --- a/arch/x86/kvm/mmu/spte.h
> +++ b/arch/x86/kvm/mmu/spte.h
> @@ -45,7 +45,9 @@ static_assert(SPTE_TDP_AD_ENABLED == 0);
>  #define ACC_EXEC_MASK    1
>  #define ACC_WRITE_MASK   PT_WRITABLE_MASK
>  #define ACC_USER_MASK    PT_USER_MASK
> -#define ACC_ALL          (ACC_EXEC_MASK | ACC_WRITE_MASK | ACC_USER_MASK)
> +#define ACC_USER_EXEC_MASK (1ULL << 3)
> +#define ACC_ALL          (ACC_EXEC_MASK | ACC_WRITE_MASK | ACC_USER_MASK | \
> +			  ACC_USER_EXEC_MASK)

This is very subtly a massive change, and I'm not convinced its one we want to
make.  All usage in the non-nested TDP flows is arguably wrong, because KVM should
never enable MBEC when using non-nested TDP.

And the use in kvm_calc_shadow_ept_root_page_role() is wrong, because the root
page role shouldn't include ACC_USER_EXEC_MASK if the associated VMCS doesn't
have MBEC.  Ditto for the use in kvm_calc_cpu_role().

So I'm pretty sure the only bit of this change that is desriable/correct is the
usage in kvm_mmu_page_get_access().  (And I guess maybe trace_mark_mmio_spte()?)

Off the cuff, I don't know what the best approach is.  One thought would be to
prep for adding ACC_USER_EXEC_MASK to ACC_ALL by introducing ACC_RWX and using
that where KVM really just wants to set RWX permissions.  That would free up
ACC_ALL for the few cases where KVM really truly wants to capture all access bits.