Re: [PATCH V2 1/5] ara virt interface of perf to support kvm guestos statistics collection in guest os

[Jes Sorensen]

On 06/23/10 03:13, Zhang, Yanmin wrote:
> On Tue, 2010-06-22 at 09:58 +0200, Jes Sorensen wrote:
>> Exposing the counters read-only would save a lot of overhead for sure.
>>>> Question is if it is safe to drop overflow support?
>>> Not safe. One of PMU hardware design objectives is to use interrupt or NMI to notify
>>> software when event counter overflows. Without overflow support, software need poll
>>> the PMU registers looply. That is not good and consumes more cpu resources.
>>
>> Here is an idea, how about having the overflow NMI in the host trigger a
>> flag that causes the PMU register read to trap and get special handling?
>> That way you could propagate the overflow back down to the guest.
> That doesn't resolve the issue that guest os software has to poll register.

That is true, but it could set a flag through the para virt interface.

Cheers,
Jes
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