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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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


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