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SubjectRe: [PATCH 1/2] KVM: x86: simplify handling of PKRU
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On 2017/8/24 17:19, Paolo Bonzini wrote:
> On 24/08/2017 11:09, Yang Zhang wrote:
>>> + if (static_cpu_has(X86_FEATURE_OSPKE) &&
>>
>> We expose protection key to VM without check whether OSPKE is enabled or
>> not. Why not check guest's cpuid here which also can avoid unnecessary
>> access to pkru?
>
> Checking guest CPUID is pretty slow. We could check CR4.PKE though.
>
> Also, using static_cpu_has with OSPKE is probably wrong. But if we do
> check CR4.PKE, we can check X86_FEATURE_PKU instead, so something like
>
> if (static_cpu_has(X86_FEATURE_PKU) &&
> kvm_read_cr4_bits(vcpu, X86_CR4_PKE) &&
> vcpu->arch.pkru != vmx->host_pkru)
>
> ... but then, kvm_read_cr4_bits is also pretty slow---and we don't
> really need it, since all CR4 writes cause a vmexit. So for now I'd
> stay with this patch, only s/static_cpu_has/boot_cpu_has/g.
>
> Of course you can send improvements on top!

ok, since most OS distributions don't support protection key so far
which means vcpu->arch.pkru always 0 in it and i remember host_pkru will
be set to 55555554 be default. To avoid the unnecessary access to pkru,
how about the following change:

@@ -4338,6 +4331,9 @@ static int vmx_set_cr4(struct kvm_vcpu *vcpu,
unsigned long cr4)
return 1;
}

+ if (cr4 & X86_CR4_PKE)
+ to_vmx(vcpu)->guest_pkru_valid = true;
+
if (to_vmx(vcpu)->nested.vmxon && !nested_cr4_valid(vcpu, cr4))
return 1;

@@ -9020,8 +9016,10 @@ static void __noclone vmx_vcpu_run(struct
kvm_vcpu *vcpu)
if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)
vmx_set_interrupt_shadow(vcpu, 0);

- if (vmx->guest_pkru_valid)
- __write_pkru(vmx->guest_pkru);
+ if (static_cpu_has(X86_FEATURE_OSPKE) &&
+ vmx->guest_pkru_valid &&
+ vcpu->arch.pkru != vmx->host_pkru)
+ __write_pkru(vcpu->arch.pkru);

atomic_switch_perf_msrs(vmx);
debugctlmsr = get_debugctlmsr();
@@ -9169,13 +9167,11 @@ static void __noclone vmx_vcpu_run(struct
kvm_vcpu *vcpu)
* back on host, so it is safe to read guest PKRU from current
* XSAVE.
*/
- if (boot_cpu_has(X86_FEATURE_OSPKE)) {
- vmx->guest_pkru = __read_pkru();
- if (vmx->guest_pkru != vmx->host_pkru) {
- vmx->guest_pkru_valid = true;
+ if (static_cpu_has(X86_FEATURE_OSPKE) &&
+ vmx->guest_pkru_valid) {
+ vcpu->arch.pkru = __read_pkru();
+ if (vcpu->arch.pkru != vmx->host_pkru)
__write_pkru(vmx->host_pkru);
- } else
- vmx->guest_pkru_valid = false;
}

/*
--
Yang
Alibaba Cloud Computing

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