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Re: [Xen-devel] tsc_mode == 1 and hvm guests

To: Stefano Stabellini <Stefano.Stabellini@xxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] tsc_mode == 1 and hvm guests
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Thu, 6 May 2010 13:04:40 +0100
Cc: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
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Should Cc Dan on this, as he's the original implementer.


On 06/05/2010 12:41, "Stefano Stabellini" <Stefano.Stabellini@xxxxxxxxxxxxx>

> Hi all,
> going through the vtsc code in xen I realized that vtsc_offset and, more
> importantly, vtsc_to_ns/ns_to_vtsc are never used for hvm guests.
> That means that if tsc_mode == 1 the rdtsc values returned to pv guests
> and the ones returns to hvm guests are fundamentally different because
> in the first case they are scaled by the guest tsc frequency and in the
> second case they are not.
> The fact that noone at the moment is calling tsc_set_info with gtsc_khz
> != 0 doesn't help spotting this problem.
> Is there a reason for this or is it just a bug?
> Also there are two gtsc_khz AFAICT: d->arch.tsc_khz and
> d->arch.hvm_domain.gtsc_khz, why hvm guests have their own separate
> record of the guest tsc frequency, when they never actually change it?
> BTW I noticed these problems because they prevent the pvclock algorithm
> from working correctly in a PV on HVM linux guest.
> Thanks,
> Stefano
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