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

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.

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).


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.


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