Re: [Xen-devel] [PATCH] x86 hvm: freeze PIT/LAPIC timer emulation while its IRQ is masked

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

Do you get a measurable performance win from this patch?

Not using pt_irq_masked() seems odd. Is it to avoid 859 IRQ mask checks, and
if so why are you doing that? Why do we care that rombios twiddles the 8259
masks a bunch during boot?

 -- Keir

On 10/09/2009 09:22, "Kouya Shimura" <kouya@xxxxxxxxxxxxxx> wrote:

> Well, this patch has two aims.  One is to stop the xen's timer by
> stop_timer(). And another is to simplify pt_update_irq() which
> searches the earliest platform timer.
> 
> To answer the latter aim, this patch gets rid of unused (masked
> by IRQ) platform timer from vcpu->arch.hvm_vcpu.tm_list.
> Your suggestion satisfies the first but the latter, I think.
> Besides, pt_update_irq() is more critical since it is called on
> every VM_EXIT.
> 
> Although I understand your uneasiness, is there any other way?
> 
> Thanks,
> Kouya
> 
> Keir Fraser writes:
>> Although I can see the sense here, the patch makes me a bit uneasy. Also I
>> think neater code would result from simply making vpt.c's handling of
>> periodic timers less stupid. By this I mean that calling set_timer() from
>> the timer handler is not really necessary -- you already know an interrupt
>> is now pending and the vcpu is kicked. May as well not set_timer() until the
>> pending interrupt(s) are delivered. That would then effectively get your
>> optimisation "for free", wouldn't it?
>> 
>>  -- Keir
>> 
>> On 10/09/2009 06:47, "Kouya Shimura" <kouya@xxxxxxxxxxxxxx> wrote:
>> 
>>> Hi,
>>> 
>>> I've found that modern windows OS never use the PIT timer,
>>> and neither cpu#0's LAPIC timer after boot.
>>> Despite that, xen emulates them busily. It's inefficient.
>>> 
>>> Note: this patch ignores the IRQ mask of legacy i8259 since
>>> rombios frequently modifies it.
>>> 
>>> Thanks,
>>> Kouya
>>> 
>>> Signed-off-by: Kouya Shimura <kouya@xxxxxxxxxxxxxx>
>>> 
>> 



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