linux-kernel - Re: [PATCH] KVM: VMX: Remove unnecessary INVEPT[GLOBAL] from hardware enable path

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Date: Thu, 20 Jun 2024 20:19:46 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: VMX: Remove unnecessary INVEPT[GLOBAL] from hardware
 enable path

On Sat, Jun 8, 2024 at 2:10 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Remove the completely pointess global INVEPT, i.e. EPT TLB flush, from
> KVM's VMX enablement path.  KVM always does a targeted TLB flush when
> using a "new" EPT root, in quotes because "new" simply means a root that
> isn't currently being used by the vCPU.
>
> KVM also _deliberately_ runs with stale TLB entries for defunct roots,
> i.e. doesn't do a TLB flush when vCPUs stop using roots, precisely because
> KVM does the flush on first use.  As called out by the comment in
> kvm_mmu_load(), the reason KVM flushes on first use is because KVM can't
> guarantee the correctness of past hypervisors.
>
> Jumping back to the global INVEPT, when the painfully terse commit
> 1439442c7b25 ("KVM: VMX: Enable EPT feature for KVM") was added, the
> effective TLB flush being performed was:
>
>   static void vmx_flush_tlb(struct kvm_vcpu *vcpu)
>   {
>           vpid_sync_vcpu_all(to_vmx(vcpu));
>   }
>
> I.e. KVM was not flushing EPT TLB entries when allocating a "new" root,
> which very strongly suggests that the global INVEPT during hardware
> enabling was a misguided hack that addressed the most obvious symptom,
> but failed to fix the underlying bug.

Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>

Paolo

> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 3 ---
>  1 file changed, 3 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 0e3aaf520db2..21dbe20f50ba 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2832,9 +2832,6 @@ int vmx_hardware_enable(void)
>                 return r;
>         }
>
> -       if (enable_ept)
> -               ept_sync_global();
> -
>         return 0;
>  }
>
>
> base-commit: af0903ab52ee6d6f0f63af67fa73d5eb00f79b9a
> --
> 2.45.2.505.gda0bf45e8d-goog
>