linux-kernel - Re: [PATCH 1/1] KVM: pass through CPUID(0x80000006)

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Message-ID: <20200415025105.GE12547@linux.intel.com>
Date:   Tue, 14 Apr 2020 19:51:05 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jon Cargille <jcargill@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Eric Northup <digitaleric@...il.com>,
        Eric Northup <digitaleric@...gle.com>
Subject: Re: [PATCH 1/1] KVM: pass through CPUID(0x80000006)

On Tue, Apr 14, 2020 at 07:37:26PM -0700, Sean Christopherson wrote:
> On Tue, Apr 14, 2020 at 06:23:20PM -0700, Jon Cargille wrote:
> > From: Eric Northup <digitaleric@...il.com>
> > 
> > Return L2 cache and TLB information to guests.
> > They could have been set before, but the defaults that KVM returns will be
> > necessary for usermode that doesn't supply their own CPUID tables.
> 
> I don't follow the changelog.  The code makes sense, but I don't understand
> the justification.  This only affects KVM_GET_SUPPORTED_CPUID, i.e. what's
> advertised to userspace, it doesn't directly change CPUID emulation in any
> way.  The "They could have been set before" blurb is especially confusing.
> 
> I assume you want to say something like:
> 
>   Return the host's L2 cache and TLB information for CPUID.0x80000006
>   instead of zeroing out the entry as part of KVM_GET_SUPPORTED_CPUID.
>   This allows a userspace VMM to feed KVM_GET_SUPPORTED_CPUID's output
>   directly into KVM_SET_CPUID2 (without breaking the guest).
> 
> > Signed-off-by: Eric Northup <digitaleric@...gle.com>
> > Signed-off-by: Eric Northup <digitaleric@...il.com>
> > Signed-off-by: Jon Cargille <jcargill@...gle.com>
> > Signed-off-by: Jim Mattson <jmattson@...gle.com>
> 
> Jim's tag is unnecessary, unless he was a middleman between Eric and Jon,
> in which case Jim's tag should also come between Eric's and Jon's.
> 
> Only one of Eric's signoffs is needed (the one that matches the From: tag,
> i.e. is the official author).  I'm guessing Google would prefer the author
> to be the @google.com address.

Ah, Eric's @google.com mail bounced.  Maybe do:

  Signed-off-by: Eric Northup (Google) <digitaleric@...il.com>

to clarify the work was done for Google without having a double signoff
and/or a dead email.