linux-kernel - Re: [PATCH 2/2] KVM: x86: Manually retrieve CPUID.0x1 when getting FMS for RESET/INIT

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Message-ID: <YVXTnheIB6MCKGve@google.com>
Date:   Thu, 30 Sep 2021 15:11:26 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        syzbot+f3985126b746b3d59c9d@...kaller.appspotmail.com,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH 2/2] KVM: x86: Manually retrieve CPUID.0x1 when getting
 FMS for RESET/INIT

On Thu, Sep 30, 2021, Paolo Bonzini wrote:
> On 30/09/21 00:24, Sean Christopherson wrote:
> >  	 * RESET since KVM emulates RESET before exposing the vCPU to userspace,
> >  	 * i.e. it'simpossible for kvm_cpuid() to find a valid entry on RESET.
> > +	 * But, go through the motions in case that's ever remedied.  Note, the
> > +	 * index for CPUID.0x1 is not significant, arbitrarily specify '0'.
> 
> Just one nit, this comment change is not really needed because almost all
> callers are using '0' for the same reason.
>
> But, perhaps adding kvm_find_cpuid_entry_index and removing the last
> parameter from kvm_find_cpuid_entry would be a good idea.

I like this idea, but only if callers are forced to specify the index when the
index is significant, e.g. add a magic CPUID_INDEX_DONT_CARE and WARN in
cpuid_entry2_find() if index is significant and index == DONT_CARE.

I'll fiddle with this, unless you want the honors?

> Also, the kvm_cpuid() reference needs to be changed, which I did upon
> commit.

Doh, thanks!