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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: Mon, 18 Oct 2010 09:34:05 -0700
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Tim Deegan <Tim.Deegan@xxxxxxxxxx>, Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
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 On 10/17/2010 06:29 PM, Dan Magenheimer wrote:
>> From: Jeremy Fitzhardinge [mailto:jeremy@xxxxxxxx]
>> Sent: Friday, October 15, 2010 6:28 PM
>> To: Dan Magenheimer
>> Cc: Tim Deegan; Stefano Stabellini; xen-devel@xxxxxxxxxxxxxxxxxxx
>> Subject: Re: pvclock (PV and HVM) and vsyscall
>>  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.
> I think (but am not positive) that the circumstances under which
> vsyscall pvclock can be enabled are exactly the same as those
> for which tsc is globally stable.  And when tsc is globally stable,
> upstream guest kernels can use tsc instead of pvclock.

Not wanting to go around in circles, but usermode can't reliably know
whether the tsc is stable enough to be usable directly.  Its most
reliable course is to always use the proper APIs and rely on the kernel
to implement the most efficient mechanism for those APIs.


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