Re: [Xen-devel] [PATCH] Always save/restore performance counters when HVM guest switching VCPU

[Suravee Suthikulanit <suravee.suthikulpanit@xxxxxxx>]

On 3/8/2013 2:47 AM, Jan Beulich wrote:
> > > > On 04.03.13 at 13:42, George Dunlap <George.Dunlap@xxxxxxxxxxxxx> wrote:
> > On Fri, Mar 1, 2013 at 8:49 PM,  <suravee.suthikulpanit@xxxxxxx> wrote:
> > > From: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
> > >
> > > Currently, the performance counter registers are saved/restores
> > > when the HVM guest switchs VCPUs only if they are running.
> > > However, PERF has one check where it writes the MSR and read back
> > > the value to check if the MSR is working.  This has shown to fails
> > > the check if the VCPU is moved in between rdmsr and wrmsr and
> > > resulting in the values are different.
> > Many moons ago (circa 2005) when I used performance counters, I found
> > that adding them to the save/restore path added a non-neligible
> > overhead -- something like 5% slow-down.  Do you have any reason to
> > believe this is no longer the case?  Have you done any benchmarks
> > before and after?
> >
> > If there is a performance slow-down, you may have to implement
> > something like the "lazy FPU" save/restore, where you remove access to
> > the VPMU MSRs to detect that the guest is accessing them.
> Suravee,
>
> without addressing George's concerns, I don't think you can
> expect the patch to be committed (the more that Boris, along
> with his ack, also asked to adjust the description).
>
> Jan
I understand that we don't want to introduce this overhead.  Let me look 
into:
1. Measuring the overhead in this case.
2. Looking into the alternative approach (lazy save/restore)

and get back to you all.

Suravee



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