Re: [PATCH 04/22] KVM: x86/mmu: Skip emulation on page fault iff 1+ SPs were unprotected

[Paolo Bonzini <pbonzini@redhat.com>]

On 8/9/24 21:03, Sean Christopherson wrote:
> When doing "fast unprotection" of nested TDP page tables, skip emulation
> if and only if at least one gfn was unprotected, i.e. continue with
> emulation if simply resuming is likely to hit the same fault and risk
> putting the vCPU into an infinite loop.
> 
> Note, it's entirely possible to get a false negative, e.g. if a different
> vCPU faults on the same gfn and unprotects the gfn first, but that's a
> relatively rare edge case, and emulating is still functionally ok, i.e.
> the risk of putting the vCPU isn't an infinite loop isn't justified.

English snafu - "the risk of causing a livelock for the vCPU is 
negligible", perhaps?

Paolo

> Fixes: 147277540bbc ("kvm: svm: Add support for additional SVM NPF error codes")
> Cc: stable@vger.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   arch/x86/kvm/mmu/mmu.c | 28 ++++++++++++++++++++--------
>   1 file changed, 20 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index e3aa04c498ea..95058ac4b78c 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5967,17 +5967,29 @@ static int kvm_mmu_write_protect_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>   	bool direct = vcpu->arch.mmu->root_role.direct;
>   
>   	/*
> -	 * Before emulating the instruction, check if the error code
> -	 * was due to a RO violation while translating the guest page.
> -	 * This can occur when using nested virtualization with nested
> -	 * paging in both guests. If true, we simply unprotect the page
> -	 * and resume the guest.
> +	 * Before emulating the instruction, check to see if the access may be
> +	 * due to L1 accessing nested NPT/EPT entries used for L2, i.e. if the
> +	 * gfn being written is for gPTEs that KVM is shadowing and has write-
> +	 * protected.  Because AMD CPUs walk nested page table using a write
> +	 * operation, walking NPT entries in L1 can trigger write faults even
> +	 * when L1 isn't modifying PTEs, and thus result in KVM emulating an
> +	 * excessive number of L1 instructions without triggering KVM's write-
> +	 * flooding detection, i.e. without unprotecting the gfn.
> +	 *
> +	 * If the error code was due to a RO violation while translating the
> +	 * guest page, the current MMU is direct (L1 is active), and KVM has
> +	 * shadow pages, then the above scenario is likely being hit.  Try to
> +	 * unprotect the gfn, i.e. zap any shadow pages, so that L1 can walk
> +	 * its NPT entries without triggering emulation.  If one or more shadow
> +	 * pages was zapped, skip emulation and resume L1 to let it natively
> +	 * execute the instruction.  If no shadow pages were zapped, then the
> +	 * write-fault is due to something else entirely, i.e. KVM needs to
> +	 * emulate, as resuming the guest will put it into an infinite loop.
>   	 */
>   	if (direct &&
> -	    (error_code & PFERR_NESTED_GUEST_PAGE) == PFERR_NESTED_GUEST_PAGE) {
> -		kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(cr2_or_gpa));
> +	    (error_code & PFERR_NESTED_GUEST_PAGE) == PFERR_NESTED_GUEST_PAGE &&
> +	    kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(cr2_or_gpa)))
>   		return RET_PF_FIXED;
> -	}
>   
>   	/*
>   	 * The gfn is write-protected, but if emulation fails we can still