linux-kernel - Re: [PATCH] kvm: x86: Use KVM CPU capabilities to determine CR4 reserved bits

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Message-ID: <6a4daca4-6034-901a-261f-215df7d606a6@intel.com>
Date:   Mon, 18 May 2020 12:52:30 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     jmattson@...gle.com,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH] kvm: x86: Use KVM CPU capabilities to determine CR4
 reserved bits

On 5/6/2020 5:44 PM, Paolo Bonzini wrote:
> Using CPUID data can be useful for the processor compatibility
> check, but that's it.  Using it to compute guest-reserved bits
> can have both false positives (such as LA57 and UMIP which we
> are already handling) and false negatives: 

> in particular, with
> this patch we don't allow anymore a KVM guest to set CR4.PKE
> when CR4.PKE is clear on the host.

A common question about whether a feature can be exposed to guest:

Given a feature, there is a CPUID bit to enumerate it, and a CR4 bit to 
turn it on/off. Whether the feature can be exposed to guest only depends 
on host CR4 setting? I.e., if CPUID bit is not cleared in cpu_data in 
host but host kernel doesn't set the corresponding CR4 bit to turn it 
on, we cannot expose the feature to guest. right?