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

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Message-ID: <894222a1-1a5d-7b04-35ae-cc3c4fa99c9a@redhat.com>
Date:   Tue, 11 Sep 2018 16:00:21 +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 15:55, Borislav Petkov wrote:
> On Tue, Sep 11, 2018 at 01:07:06PM +0200, Paolo Bonzini wrote:
>> If the host TSCs are unsynchronized then yes, that's what happens.  And
>> you can do live migration from synchronized to unsynchronized.
> 
> Which brings us back to my original question: why would we *ever* want
> to support unsynchronized TSCs in a guest? Such machines are a real
> abomination for baremetal - it doesn't make *any* sense to me to have
> that in guests too, if it can be helped...

No, wait.  The host TSC is unsynchronized, _so_ you need one kvmclock
struct per vCPU.  The resulting kvmclock is synchronized.

Paolo