Re: [Xen-devel] [PATCH v2] x86/hvm: re-work viridian APIC assist code

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: David Woodhouse [mailto:dwmw2@xxxxxxxxxxxxx]
> Sent: 04 September 2018 21:31
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Jan Beulich
> <jbeulich@xxxxxxxx>; Eslam Elnikety <elnikety@xxxxxxxxx>; Shan Haitao
> <haitao.shan@xxxxxxxxx>
> Subject: Re: [Xen-devel] [PATCH v2] x86/hvm: re-work viridian APIC assist
> code
> 
> On Mon, 2018-09-03 at 10:12 +0000, Paul Durrant wrote:
> >
> > I believe APIC assist is intended for fully synthetic interrupts.
> 
> Hm, if by 'fully synthetic interrupts' you mean
> vlapic_virtual_intr_delivery_enabled(), then no I think APIC assist
> doesn't get used in that case at all.

No, what I meant was that it was my belief that Windows would avoid APIC assist 
for what it sees as devices on a hardware bus but - now I think about it - that 
would not be wonderfully useful behaviour and would certainly squash the reason 
I put the support in anyway, which is for the edge triggered per-cpu event 
channel upcalls from Xen (since the driver registering the interrupts is bound 
to the Xen platform PCI device).

> 
> > Is it definitely this patch that causes the problem? It was only
> > intended to fix previous incorrectness but, if this is the culprit,
> > then it's clearly caused collateral damage in a logically unrelated
> > area.
> 
> Not entirely. The performance gain we observed with APIC assist in the
> first place was basically stolen. It wasn't just bypassing the vmexit
> for that EOI; it was *so* much faster because it actually didn't ever
> do the EOI properly at all.
> 
> You fixed that omission and unsurprisingly it got slower again; most of
> the apparent benefit of APIC assist is lost. But that's because it was
> never really doing the right thing in the first place.
> 
> That EOI handling for unmaskable MSI is really painfully slow, so my
> hack bypasses it in the common case where it isn't really necessary.
> FWIW I've done it in my tree with a single per-domain flag rather than
> a per-vector bitmap now, which makes it slightly simpler.

I see. Given that Windows has used APIC assist to circumvent its EOI then I 
wonder whether we can get away with essentially doing the same. I.e. for a 
completed APIC assist found in vlapic_has_pending_irq() we simply clear the 
APIC assist and highest vector in the ISR, rather than calling through to 
vlapic_EOI_set() and suffering the overhead. I'll spin up a patch and give it a 
whirl.

  Paul
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