linux-kernel - Re: [PATCH v6 5/5] x86/kvm: Avoid dynamic allocation of pvclock data when SEV is active

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Date:   Tue, 11 Sep 2018 12:19:13 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Borislav Petkov <bp@...e.de>
Cc:     Brijesh Singh <brijesh.singh@....com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v6 5/5] x86/kvm: Avoid dynamic allocation of pvclock data
 when SEV is active

On 11/09/2018 12:01, Borislav Petkov wrote:
> On Tue, Sep 11, 2018 at 11:26:21AM +0200, Paolo Bonzini wrote:
>> Usually the kvmclock structs are all the same, but there is support for
>> old machines with inconsistent TSCs (across different sockets typically).
> 
> Would that be a problem, though? Sounds like an "improvement" to me. :-)
> 
> I mean, if we keep using the same TSC across all vCPUs, the guest will
> actually see a single TSC and thus have stable and synchronized TSCs.
> Unlike the host.

That's exactly what kvmclock is for, it provides a stable and
synchronized clock on top of unsynchronized TSCs.  But that's also why
you need one struct per vCPU, at least in the synchronized case.

Paolo

> I.e., the guest will be better than the host! :-)
>