Re: [Qemu-devel] Para-Virtualized Clock Usage

[Gleb Natapov <gleb@redhat.com>]

On Wed, May 15, 2013 at 08:40:25PM +0000, Joji Mekkattuparamban (joji) wrote:
> Hi Gleb,
> 
> On a related note, does Qemu have an option to emulate RDTSC?
> 
QEMU or KVM? With KVM guest executes RDTSC directly on CPU.

> Thanks,
> Joji.
> 
> -----Original Message-----
> From: Gleb Natapov [mailto:gleb@redhat.com] 
> Sent: Wednesday, April 24, 2013 11:57 PM
> To: Joji Mekkattuparamban (joji)
> Cc: Marcelo Tosatti; qemu-devel@nongnu.org; kvm@vger.kernel.org
> Subject: Re: [Qemu-devel] Para-Virtualized Clock Usage
> 
> On Thu, Apr 25, 2013 at 12:28:35AM +0000, Joji Mekkattuparamban (joji) wrote:
> > Thank you Gleb and Marcelo. I will migrate the API using gettimeofday.
> > 
> > Is there any dependency on the QEMU or the Guest? If the host supports pvclock and the guest invokes gettimeofday, would the pvclock be automatically used? Or do I require a patch in either the Qemu or the guest kernel?
> > 
> Guest and host kernel should be at least 3.8. IIRC there is not QEMU version dependency.
> 
> > Thanks!
> > Joji.
> > 
> > -----Original Message-----
> > From: Marcelo Tosatti [mailto:mtosatti@redhat.com]
> > Sent: Wednesday, April 24, 2013 1:28 AM
> > To: Gleb Natapov
> > Cc: Joji Mekkattuparamban (joji); qemu-devel@nongnu.org; 
> > kvm@vger.kernel.org
> > Subject: Re: [Qemu-devel] Para-Virtualized Clock Usage
> > 
> > On Tue, Apr 23, 2013 at 08:52:16AM +0300, Gleb Natapov wrote:
> > > On Mon, Apr 22, 2013 at 04:58:01PM +0000, Joji Mekkattuparamban (joji) wrote:
> > > > Greetings,
> > > > 
> > > > I have a SMP guest application, running on the 2.6.27 Linux kernel. The application, originally written for bare metal, makes extensive use of the TSC, by directly invoking rdtsc from the user space for timestamp purposes. While running on KVM (RHEL version 6.3), we are running into TSC issues on some hardware. As a solution, I am considering migrating to the pvclock. I am wondering if there is an example for migrating from TSC to the pvclock. Any pointers?
> > > > 
> > > Wrong list, you should ask KVM (copied). Recent kernels have pvclock 
> > > vdso support which means that gettimeofday() uses it without 
> > > entering the kernel. Marcelo?
> > > 
> > > --
> > > 			Gleb.
> > 
> > Converting application to make use of gettimeofday() should be the best way to make use of pvclock, yes.
> 
> --
> 			Gleb.
> --
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--
			Gleb.