Re: [PATCH] KVM: x86: VMX: Make smaller physical guest address space support user-configurable

[Naresh Kamboju]

Hi Paolo,

Thanks for the patch.

On Tue, 29 Sep 2020 at 20:17, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On 29/09/20 15:39, Qian Cai wrote:
> > On Tue, 2020-09-29 at 14:26 +0200, Paolo Bonzini wrote:
> >> On 29/09/20 13:59, Qian Cai wrote:
> >>> WARN_ON_ONCE(!allow_smaller_maxphyaddr);
> >>>
> >>> I noticed the origin patch did not have this WARN_ON_ONCE(), but the
> >>> mainline
> >>> commit b96e6506c2ea ("KVM: x86: VMX: Make smaller physical guest address
> >>> space
> >>> support user-configurable") does have it for some reasons.
> >>
> >> Because that part of the code should not be reached.  The exception
> >> bitmap is set up with
> >>
> >>         if (!vmx_need_pf_intercept(vcpu))
> >>                 eb &= ~(1u << PF_VECTOR);
> >>
> >> where
> >>
> >> static inline bool vmx_need_pf_intercept(struct kvm_vcpu *vcpu)
> >> {
> >>         if (!enable_ept)
> >>                 return true;
> >>
> >>         return allow_smaller_maxphyaddr &&
> >>               cpuid_maxphyaddr(vcpu) < boot_cpu_data.x86_phys_bits;
> >> }
> >>
> >> We shouldn't get here if "enable_ept && !allow_smaller_maxphyaddr",
> >> which implies vmx_need_pf_intercept(vcpu) == false.  So the warning is
> >> genuine; I've sent a patch.
> >
> > Care to provide a link to the patch? Just curious.
> >
>
> Ok, I haven't sent it yet. :)  But here it is:
>
> commit 608e2791d7353e7d777bf32038ca3e7d548155a4 (HEAD -> kvm-master)
> Author: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Date:   Tue Sep 29 08:31:32 2020 -0400
>
>     KVM: VMX: update PFEC_MASK/PFEC_MATCH together with PF intercept
>
>     The PFEC_MASK and PFEC_MATCH fields in the VMCS reverse the meaning of
>     the #PF intercept bit in the exception bitmap when they do not match.
>     This means that, if PFEC_MASK and/or PFEC_MATCH are set, the
>     hypervisor can get a vmexit for #PF exceptions even when the
>     corresponding bit is clear in the exception bitmap.
>
>     This is unexpected and is promptly reported as a WARN_ON_ONCE.
>     To fix it, reset PFEC_MASK and PFEC_MATCH when the #PF intercept
>     is disabled (as is common with enable_ept && !allow_smaller_maxphyaddr).

I have tested this patch on an x86_64 machine and the reported issue is gone.

>
>     Reported-by: Qian Cai <cai@xxxxxxxxxx>
>     Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
Tested-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>

>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index f0384e93548a..f4e9c310032a 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -794,6 +794,18 @@ void update_exception_bitmap(struct kvm_vcpu *vcpu)
>          */
>         if (is_guest_mode(vcpu))
>                 eb |= get_vmcs12(vcpu)->exception_bitmap;
> +        else {
> +               /*
> +                * If EPT is enabled, #PF is only trapped if MAXPHYADDR is mismatched
> +                * between guest and host.  In that case we only care about present
> +                * faults.  For vmcs02, however, PFEC_MASK and PFEC_MATCH are set in
> +                * prepare_vmcs02_rare.
> +                */
> +               bool selective_pf_trap = enable_ept && (eb & (1u << PF_VECTOR));
> +               int mask = selective_pf_trap ? PFERR_PRESENT_MASK : 0;
> +               vmcs_write32(PAGE_FAULT_ERROR_CODE_MASK, mask);
> +               vmcs_write32(PAGE_FAULT_ERROR_CODE_MATCH, mask);
> +       }
>
>         vmcs_write32(EXCEPTION_BITMAP, eb);
>  }
> @@ -4355,16 +4367,6 @@ static void init_vmcs(struct vcpu_vmx *vmx)
>                 vmx->pt_desc.guest.output_mask = 0x7F;
>                 vmcs_write64(GUEST_IA32_RTIT_CTL, 0);
>         }
> -
> -       /*
> -        * If EPT is enabled, #PF is only trapped if MAXPHYADDR is mismatched
> -        * between guest and host.  In that case we only care about present
> -        * faults.
> -        */
> -       if (enable_ept) {
> -               vmcs_write32(PAGE_FAULT_ERROR_CODE_MASK, PFERR_PRESENT_MASK);
> -               vmcs_write32(PAGE_FAULT_ERROR_CODE_MATCH, PFERR_PRESENT_MASK);
> -       }
>  }
>
>  static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>

test log link
https://lkft.validation.linaro.org/scheduler/job/1813223

- Naresh