linux-kernel - Re: [PATCH 2/6] KVM: VMX: remove bogus check for invalid EPT violation

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Date:   Thu, 30 Mar 2017 09:30:16 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 2/6] KVM: VMX: remove bogus check for invalid EPT violation

On Thu, Mar 30, 2017 at 2:55 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> handle_ept_violation is checking for "guest-linear-address invalid" +
> "not a paging-structure walk".  However, _all_ EPT violations without
> a valid guest linear address are paging structure walks, because those
> EPT violations happen when loading the guest PDPTEs.
>
> Therefore, the check can never be true, and even if it were, KVM doesn't
> care about the guest linear address; it only uses the guest *physical*
> address VMCS field.  So, remove the check altogether.
>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
The check can never be true because the SDM says so explicitly: Bit 8
is "Reserved if bit 7 is 0 (cleared to 0)."

Reviewed-by: Jim Mattson <jmattson@...gle.com>
> ---
>  arch/x86/kvm/vmx.c | 14 --------------
>  1 file changed, 14 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 0e61b9226bf2..1c372600a962 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -6208,23 +6208,9 @@ static int handle_ept_violation(struct kvm_vcpu *vcpu)
>         unsigned long exit_qualification;
>         gpa_t gpa;
>         u32 error_code;
> -       int gla_validity;
>
>         exit_qualification = vmcs_readl(EXIT_QUALIFICATION);
>
> -       gla_validity = (exit_qualification >> 7) & 0x3;
> -       if (gla_validity == 0x2) {
> -               printk(KERN_ERR "EPT: Handling EPT violation failed!\n");
> -               printk(KERN_ERR "EPT: GPA: 0x%lx, GVA: 0x%lx\n",
> -                       (long unsigned int)vmcs_read64(GUEST_PHYSICAL_ADDRESS),
> -                       vmcs_readl(GUEST_LINEAR_ADDRESS));
> -               printk(KERN_ERR "EPT: Exit qualification is 0x%lx\n",
> -                       (long unsigned int)exit_qualification);
> -               vcpu->run->exit_reason = KVM_EXIT_UNKNOWN;
> -               vcpu->run->hw.hardware_exit_reason = EXIT_REASON_EPT_VIOLATION;
> -               return 0;
> -       }
> -
>         /*
>          * EPT violation happened while executing iret from NMI,
>          * "blocked by NMI" bit has to be set before next VM entry.
> --
> 1.8.3.1
>
>