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Re: [Qemu-devel] interrupt handling in qemu

From: Peter Maydell
Subject: Re: [Qemu-devel] interrupt handling in qemu
Date: Wed, 28 Dec 2011 01:10:41 +0000

On 28 December 2011 00:43, Xin Tong <address@hidden> wrote:
> Which version of QEMU did you do your test on, and what are the tests.

It was whatever trunk qemu was a year or so ago, and the test was
just "time to login prompt for ARM guest in system mode".

> I modified QEMU to check for interrupt status at the end of every TB
> and ran it on SPECINT2000 benchmarks with QEMU 0.15.0. The performance
> is 70% of the unmodified one for some benchmarks on a x86_64 host.

I don't suppose you could provide a brief set of instructions for
setting up a benchmark setup like this? (Is this user-mode or

> I
> agree that the extra load-test-branch-not-taken per TB is minimal, but
> what I found is that the average number of TB executed per TB enter is
> low (~3.5 TBs), while the unmodified approach has ~10 TBs per TB
> enter. this makes me wonder why. Maybe the mechanism i used to gather
> this statistics is flawed. but the performance is indeed hindered.


> By keeping a counter that decrements on every TB, and when the counter
> reaches 0, the current executing TB checks for interrupt status.

Decrementing a counter on every TB is going to be slower than
just checking a flag, so you might as well just check the flag.
(If the flag is set you need to handle the interrupt anyway
so there's no point delaying it.)

-- PMM

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