Re: [PATCH 05/16] KVM: x86/mmu: Use synthetic page fault error code to indicate private faults

[Xu Yilun]

On Tue, Feb 27, 2024 at 06:41:36PM -0800, Sean Christopherson wrote:
> Add and use a synthetic, KVM-defined page fault error code to indicate
> whether a fault is to private vs. shared memory.  TDX and SNP have
> different mechanisms for reporting private vs. shared, and KVM's
> software-protected VMs have no mechanism at all.  Usurp an error code
> flag to avoid having to plumb another parameter to kvm_mmu_page_fault()
> and friends.
> 
> Alternatively, KVM could borrow AMD's PFERR_GUEST_ENC_MASK, i.e. set it
> for TDX and software-protected VMs as appropriate, but that would require
> *clearing* the flag for SEV and SEV-ES VMs, which support encrypted
> memory at the hardware layer, but don't utilize private memory at the
> KVM layer.

I see this alternative in other patchset.  And I still don't understand why
synthetic way is better after reading patch #5-7.  I assume for SEV(-ES) if
there is spurious PFERR_GUEST_ENC flag set in error code when private memory
is not used in KVM, then it is a HW issue or SW bug that needs to be caught
and warned, and by the way cleared.

Thanks,
Yilun

> 
> Opportunistically add a comment to call out that the logic for software-
> protected VMs is (and was before this commit) broken for nested MMUs, i.e.
> for nested TDP, as the GPA is an L2 GPA.  Punt on trying to play nice with
> nested MMUs as there is a _lot_ of functionality that simply doesn't work
> for software-protected VMs, e.g. all of the paths where KVM accesses guest
> memory need to be updated to be aware of private vs. shared memory.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h | 11 +++++++++++
>  arch/x86/kvm/mmu/mmu.c          | 26 +++++++++++++++++++-------
>  arch/x86/kvm/mmu/mmu_internal.h |  2 +-
>  3 files changed, 31 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 1e69743ef0fb..4077c46c61ab 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -267,7 +267,18 @@ enum x86_intercept_stage;
>  #define PFERR_GUEST_ENC_MASK	BIT_ULL(34)
>  #define PFERR_GUEST_SIZEM_MASK	BIT_ULL(35)
>  #define PFERR_GUEST_VMPL_MASK	BIT_ULL(36)
> +
> +/*
> + * IMPLICIT_ACCESS is a KVM-defined flag used to correctly perform SMAP checks
> + * when emulating instructions that triggers implicit access.
> + */
>  #define PFERR_IMPLICIT_ACCESS	BIT_ULL(48)
> +/*
> + * PRIVATE_ACCESS is a KVM-defined flag us to indicate that a fault occurred
> + * when the guest was accessing private memory.
> + */
> +#define PFERR_PRIVATE_ACCESS	BIT_ULL(49)
> +#define PFERR_SYNTHETIC_MASK	(PFERR_IMPLICIT_ACCESS | PFERR_PRIVATE_ACCESS)
>  
>  #define PFERR_NESTED_GUEST_PAGE (PFERR_GUEST_PAGE_MASK |	\
>  				 PFERR_WRITE_MASK |		\
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 408969ac1291..7807bdcd87e8 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5839,19 +5839,31 @@ int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 err
>  	bool direct = vcpu->arch.mmu->root_role.direct;
>  
>  	/*
> -	 * IMPLICIT_ACCESS is a KVM-defined flag used to correctly perform SMAP
> -	 * checks when emulating instructions that triggers implicit access.
>  	 * WARN if hardware generates a fault with an error code that collides
> -	 * with the KVM-defined value.  Clear the flag and continue on, i.e.
> -	 * don't terminate the VM, as KVM can't possibly be relying on a flag
> -	 * that KVM doesn't know about.
> +	 * with KVM-defined sythentic flags.  Clear the flags and continue on,
> +	 * i.e. don't terminate the VM, as KVM can't possibly be relying on a
> +	 * flag that KVM doesn't know about.
>  	 */
> -	if (WARN_ON_ONCE(error_code & PFERR_IMPLICIT_ACCESS))
> -		error_code &= ~PFERR_IMPLICIT_ACCESS;
> +	if (WARN_ON_ONCE(error_code & PFERR_SYNTHETIC_MASK))
> +		error_code &= ~PFERR_SYNTHETIC_MASK;
>  
>  	if (WARN_ON_ONCE(!VALID_PAGE(vcpu->arch.mmu->root.hpa)))
>  		return RET_PF_RETRY;
>  
> +	/*
> +	 * Except for reserved faults (emulated MMIO is shared-only), set the
> +	 * private flag for software-protected VMs based on the gfn's current
> +	 * attributes, which are the source of truth for such VMs.  Note, this
> +	 * wrong for nested MMUs as the GPA is an L2 GPA, but KVM doesn't
> +	 * currently supported nested virtualization (among many other things)
> +	 * for software-protected VMs.
> +	 */
> +	if (IS_ENABLED(CONFIG_KVM_SW_PROTECTED_VM) &&
> +	    !(error_code & PFERR_RSVD_MASK) &&
> +	    vcpu->kvm->arch.vm_type == KVM_X86_SW_PROTECTED_VM &&
> +	    kvm_mem_is_private(vcpu->kvm, gpa_to_gfn(cr2_or_gpa)))
> +		error_code |= PFERR_PRIVATE_ACCESS;
> +
>  	r = RET_PF_INVALID;
>  	if (unlikely(error_code & PFERR_RSVD_MASK)) {
>  		r = handle_mmio_page_fault(vcpu, cr2_or_gpa, direct);
> diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> index 1fab1f2359b5..d7c10d338f14 100644
> --- a/arch/x86/kvm/mmu/mmu_internal.h
> +++ b/arch/x86/kvm/mmu/mmu_internal.h
> @@ -306,7 +306,7 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>  		.max_level = KVM_MAX_HUGEPAGE_LEVEL,
>  		.req_level = PG_LEVEL_4K,
>  		.goal_level = PG_LEVEL_4K,
> -		.is_private = kvm_mem_is_private(vcpu->kvm, cr2_or_gpa >> PAGE_SHIFT),
> +		.is_private = err & PFERR_PRIVATE_ACCESS,
>  	};
>  	int r;
>  
> -- 
> 2.44.0.278.ge034bb2e1d-goog
> 
>