linux-kernel - Re: [RFC v2 0/2] kvm: Use host timekeeping in guest.

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Message-ID: <CABCjUKAprLkXb4Mw0VAY9ODRD81sV0VyPZYFkuxKajcdqf67vw@mail.gmail.com>
Date:   Tue, 15 Oct 2019 14:08:46 +0900
From:   Suleiman Souhlal <suleiman@...gle.com>
To:     Roman Kagan <rkagan@...tuozzo.com>
Cc:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "john.stultz@...aro.org" <john.stultz@...aro.org>,
        "sboyd@...nel.org" <sboyd@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "ssouhlal@...ebsd.org" <ssouhlal@...ebsd.org>,
        "tfiga@...omium.org" <tfiga@...omium.org>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>, konrad.wilk@...cle.com
Subject: Re: [RFC v2 0/2] kvm: Use host timekeeping in guest.

On Thu, Oct 10, 2019 at 7:39 PM Roman Kagan <rkagan@...tuozzo.com> wrote:
>
> I wonder how feasible it is to map the host's vdso into the guest and
> thus make the guest use the *same* (as opposed to "synchronized") clock
> as the host's userspace?  Another benefit is that it's essentially an
> ABI so is not changed as liberally as internal structures like
> timekeeper, etc.  There is probably certain complication in handling the
> syscall fallback in the vdso when used in the guest kernel, though.
>
> You'll also need to ensure neither tsc scaling and nor offsetting is
> applied to the VM once this clock is enabled.

That is what I initially wanted to do, but I couldn't find an easy way
to map a host page into the guest, outside of the regular userspace
(ioctl) KVM way of adding memory to a VM.

-- Suleiman