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From:   Suleiman Souhlal <suleiman@google.com>
Date:   Tue, 15 Oct 2019 14:08:46 +0900
Message-ID: <CABCjUKAprLkXb4Mw0VAY9ODRD81sV0VyPZYFkuxKajcdqf67vw@mail.gmail.com>
Subject: Re: [RFC v2 0/2] kvm: Use host timekeeping in guest.
To:     Roman Kagan <rkagan@virtuozzo.com>
Cc:     "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "rkrcmar@redhat.com" <rkrcmar@redhat.com>,
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "ssouhlal@freebsd.org" <ssouhlal@freebsd.org>,
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        "vkuznets@redhat.com" <vkuznets@redhat.com>, konrad.wilk@oracle.com
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On Thu, Oct 10, 2019 at 7:39 PM Roman Kagan <rkagan@virtuozzo.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