linux-kernel - Re: [RFC 2/7] KVM: VMX: Expose IA32

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Message-ID: <192cfa2c-e54a-c7c1-30dd-7077e07e4af1@redhat.com>
Date:   Mon, 1 Feb 2021 11:05:23 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Chenyi Qiang <chenyi.qiang@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 2/7] KVM: VMX: Expose IA32_PKRS MSR

On 01/02/21 10:53, Chenyi Qiang wrote:
>>>
>>
>> Is the guest expected to do a lot of reads/writes to the MSR (e.g. at 
>> every context switch)?
>>
>> Even if this is the case, the MSR intercepts and the entry/exit 
>> controls should only be done if CR4.PKS=1.  If the guest does not use 
>> PKS, KVM should behave as if these patches did not exist.
>>
> 
> Hi Paolo,
> 
> Per the MSR intercepts and entry/exit controls, IA32_PKRS access is 
> independent of the CR4.PKS bit, it just depends on CPUID enumeration. If 
> the guest doesn't set CR4.PKS but still has the CPUID capability, 
> modifying on PKRS should be supported but has no effect. IIUC, we can 
> not ignore these controls if CR4.PKS=0.

Understood, I wanted to avoid paying the price (if any) of loading PKRS 
on vmentry and vmexit not just if CPUID.PKS=0, but also if CR4.PKS=0. 
If CR4.PKS=0 it would be nicer to enable the MSR intercept and disable 
the vmentry/vmexit controls; just run the guest with the host value of 
IA32_PKRS.

Paolo