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[Xen-devel] Re: pvclock (PV and HVM) and vsyscall

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: [Xen-devel] Re: pvclock (PV and HVM) and vsyscall
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Fri, 15 Oct 2010 17:28:25 -0700
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Tim Deegan <Tim.Deegan@xxxxxxxxxx>, Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
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 On 10/15/2010 08:48 AM, Dan Magenheimer wrote:
> The other discussion of RADclock reminded me:
> IIRC the pvclock algorithm is still incompatible with
> vsyscall/vdso (fast system calls) and there was no obvious
> and upstreamable solution to resolve this.
> This means that any userland call to the various gettimeofday
> routines will always do a true system call on both (a) a PV
> domain or (b) any PV on HVM domain with Stefanos' pvclock patch.
> Since true syscalls are very expensive on a 64-bit
> PV domain, I'm wondering if pvclock is still the right
> default choice for upstream (at least for 64-bit).

What other options are there?  If the tsc is globally stable, then using
pvclock in userspace will work fine; if it isn't, you'll need to do the
syscall anyway.

There's no basic problem with the vsyscall pvclock patch so long as we
can know under what circumstances it is safe to enable.


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