linux-kernel - Re: [PATCH] KVM: nVMX: INVPCID support

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Date:   Tue, 1 Aug 2017 13:35:31 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: nVMX: INVPCID support

On 01/08/2017 13:18, David Hildenbrand wrote:
> 
>>> Can't we rewrite that a little bit, avoiding that "best" handling
>>> (introducing guest_cpuid_disable_invpcid() and guest_cpuid_has_invpcid())
>>>
>>> bool invpcid_enabled = guest_cpuid_has_pcid(vcpu) &&
>>> 		       guest_cpuid_has_invpcid();
>>>
>>> if (!invpcid_enabled) {
>>> 	secondary_exec_ctl &= ~SECONDARY_EXEC_ENABLE_INVPCID;
>>> 	/* make sure there is no no INVPCID without PCID */
>>> 	guest_cpuid_disable_invpcid(vcpu);
>>> }
>>
>> I don't know... if you need a comment, it means the different structure
>> of the code doesn't spare many doubts from the reader.  And the code
>> doesn't become much simpler since you have to handle "nested" anyway.
>> What I tried to do was to mimic as much as possible the rdtscp case, but
>> it cannot be exactly the same due to the interaction between PCID and
>> INVPCID.
> 
> It's more about the handling of best here, which can be avoided quite
> easily as I showed (by encapsulating how cpuids are looked up/modified).

Yeah, I don't like either option. :)  Luckily there is a second maintainer!

Paolo