linux-kernel - Re: [PATCH] KVM: x86: call irq notifiers with directed EOI

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Date:	Wed, 18 Mar 2015 21:16:06 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Bandan Das <bsd@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>, chao.zhou@...el.com
Subject: Re: [PATCH] KVM: x86: call irq notifiers with directed EOI

2015-03-18 15:37-0400, Bandan Das:
> Radim Krčmář <rkrcmar@...hat.com> writes:
> > kvm_ioapic_update_eoi() wasn't called if directed EOI was enabled.
> > We need to do that for irq notifiers.  (Like with edge interrupts.)
> 
> Wow! It's interesting that this path is only hit with Xen as guest.

Linux doesn't use directed EOI ... KVM should fail with anything that
depends on PIT, so probably only Xen bothered to implement it :)

> I always thought of directed EOI as a "security feature" since broadcast
> could lead to interrupt storms (or something like that) :)

I think it is just an unpopular optimization for large systems.

(With multiple IO-APICs: IRQ handler knows which ones need the EOI, but
 LAPIC doesn't, hence we avoid some useless poking if OS does it ...
 EOI interrupt storm happens because right after EOI, the IO-APIC can
 send another interrupt and real hardware is slow, so CPU manages some
 cycles before receiving the next one, but KVM works instantaneously.)
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