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

[Kouya Shimura <kouya@xxxxxxxxxxxxxx>]

Keir Fraser writes:
> Do you get a measurable performance win from this patch?

Sorry, I don't measure yet. Probably a little bit win.

> 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?

I guessed that the cost of freezing/thawing a timer with the 8259 mask
is higher than timer emulation.  And the PIT timer is actually active
while rombios masks the 8259.

If using pt_irq_masked() is desirable, I'll remake it.

Thanks,
Kouya

> 
>  -- 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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