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: <cacb6865-e4ad-80c9-7482-23e480a61b0f@redhat.com>
Date:   Tue, 11 Sep 2018 13:07:06 +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 12:25, Borislav Petkov wrote:
> On Tue, Sep 11, 2018 at 12:19:13PM +0200, Paolo Bonzini wrote:
>> 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.
                                                 ^^^^^^^^^^^^

unsynchronized

> 
> Why?
> 
> Why can't it be a single pointer to a struct pvclock_vsyscall_time_info
> shared between all vCPUs?
> 
> Or does each vCPU write its own specific stuff into it so it has to be
> per-vCPU?

If the host TSCs are unsynchronized then yes, that's what happens.  And
you can do live migration from synchronized to unsynchronized.

Paolo