linux-kernel - Re: [PATCH 1/2] KVM: x86: simplify handling of PKRU

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Message-ID: <40adf946-79ad-87cd-8bfd-6db4dfdbefc3@redhat.com>
Date:   Thu, 24 Aug 2017 11:19:33 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Yang Zhang <yang.zhang.wz@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     junkang.fjk@...baba-inc.com
Subject: Re: [PATCH 1/2] KVM: x86: simplify handling of PKRU

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!

Paolo

>> +        vcpu->arch.pkru != vmx->host_pkru)
>> +        __write_pkru(vcpu->arch.pkru);