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

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Message-ID: <20191011185050.GJ691@char.us.oracle.com>
Date:   Fri, 11 Oct 2019 14:50:50 -0400
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Roman Kagan <rkagan@...tuozzo.com>,
        Suleiman Souhlal <suleiman@...gle.com>, pbonzini@...hat.com,
        rkrcmar@...hat.com, tglx@...utronix.de, john.stultz@...aro.org,
        sboyd@...nel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, ssouhlal@...ebsd.org, tfiga@...omium.org,
        vkuznets@...hat.com
Subject: Re: [RFC v2 0/2] kvm: Use host timekeeping in guest.

.snip..
> 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.

This is how Xen does it - you can register the hypervisor to timestamp
your vDSO regions if you want it. See xen_setup_vsyscall_time_info

> 
> Roman.