2021-02-12 00:45:14

by Sean Christopherson

Subject: [PATCH 3/3] KVM: VMX: Allow INVPCID in guest without PCID

Remove the restriction that prevents VMX from exposing INVPCID to the
guest without PCID also being exposed to the guest. The justification of
the restriction is that INVPCID will #UD if it's disabled in the VMCS.
While that is a true statement, it's also true that RDTSCP will #UD if
it's disabled in the VMCS. Neither of those things has any dependency
whatsoever on the guest being able to set CR4.PCIDE=1, which is what is
effectively allowed by exposing PCID to the guest.

Removing the bogus restriction aligns VMX with SVM, and also allows for
an interesting configuration. INVPCID is that fastest way to do a global
TLB flush, e.g. see native_flush_tlb_global(). Allowing INVPCID without
PCID would let a guest use the expedited flush while also limiting the
number of ASIDs consumed by the guest.

Signed-off-by: Sean Christopherson <[email protected]>
---
arch/x86/kvm/vmx/vmx.c | 10 ----------
1 file changed, 10 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 6d265b2523f8..e1b84008a05d 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4295,18 +4295,8 @@ static void vmx_compute_secondary_exec_control(struct vcpu_vmx *vmx)
}

vmx_adjust_sec_exec_feature(vmx, &exec_control, rdtscp, RDTSCP);
-
- /*
- * Expose INVPCID if and only if PCID is also exposed to the guest.
- * INVPCID takes a #UD when it's disabled in the VMCS, but a #GP or #PF
- * if CR4.PCIDE=0. Enumerating CPUID.INVPCID=1 would lead to incorrect
- * behavior from the guest perspective (it would expect #GP or #PF).
- */
- if (!guest_cpuid_has(vcpu, X86_FEATURE_PCID))
- guest_cpuid_clear(vcpu, X86_FEATURE_INVPCID);
vmx_adjust_sec_exec_feature(vmx, &exec_control, invpcid, INVPCID);

-
vmx_adjust_sec_exec_exiting(vmx, &exec_control, rdrand, RDRAND);
vmx_adjust_sec_exec_exiting(vmx, &exec_control, rdseed, RDSEED);

--
2.30.0.478.g8a0d178c01-goog

2021-02-12 01:06:53

by Jim Mattson

Subject: Re: [PATCH 3/3] KVM: VMX: Allow INVPCID in guest without PCID

On Thu, Feb 11, 2021 at 4:34 PM Sean Christopherson <[email protected]> wrote:
>
> Remove the restriction that prevents VMX from exposing INVPCID to the
> guest without PCID also being exposed to the guest. The justification of
> the restriction is that INVPCID will #UD if it's disabled in the VMCS.
> While that is a true statement, it's also true that RDTSCP will #UD if
> it's disabled in the VMCS. Neither of those things has any dependency
> whatsoever on the guest being able to set CR4.PCIDE=1, which is what is
> effectively allowed by exposing PCID to the guest.
>
> Removing the bogus restriction aligns VMX with SVM, and also allows for
> an interesting configuration. INVPCID is that fastest way to do a global
> TLB flush, e.g. see native_flush_tlb_global(). Allowing INVPCID without
> PCID would let a guest use the expedited flush while also limiting the
> number of ASIDs consumed by the guest.
>
> Signed-off-by: Sean Christopherson <[email protected]>
I always thought this was a bizarre one-off restriction.
Reviewed-by: Jim Mattson <[email protected]>