From: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
To: Peter Maydell <peter.maydell@linaro.org>,
Programmingkid <programmingkidx@gmail.com>
Cc: Richard Henderson <richard.henderson@linaro.org>,
QEMU devel list <qemu-devel@nongnu.org>
Subject: Re: tb_flush() calls causing long Windows XP boot times
Date: Thu, 10 Jun 2021 14:24:13 +0100 [thread overview]
Message-ID: <1ee4b7cf-d445-6497-705f-510009fc74f8@ilande.co.uk> (raw)
In-Reply-To: <CAFEAcA-YuyZ9kyivh1dL_chxrtvBF=jOf3L59JuroL2U-e+Xsg@mail.gmail.com>
On 10/06/2021 14:14, Peter Maydell wrote:
> On Thu, 10 Jun 2021 at 14:02, Programmingkid <programmingkidx@gmail.com> wrote:
>>
>> Hi Richard,
>>
>> There is a function called breakpoint_invalidate() in cpu.c that calls a function called tb_flush(). I have determined that this call is being made over 200,000 times when Windows XP boots. Disabling this function makes Windows XP boot way faster than before. The time went down from around 3 minutes to 20 seconds when I applied the patch below.
>>
>> After I applied the patch I ran several tests in my VM's to see if anything broke. I could not find any problems. Here is the list my VM's I tested:
>>
>> Mac OS 10.8 in qemu-system-x86_64
>> Windows 7 in qemu-system-x86_64
>> Windows XP in qemu-system-i386
>> Mac OS 10.4 in qemu-system-ppc
>>
>> I would be happy if the patch below was accepted but I would like to know your thoughts.
>
>> cpu.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/cpu.c b/cpu.c
>> index bfbe5a66f9..297c2e4281 100644
>> --- a/cpu.c
>> +++ b/cpu.c
>> @@ -253,7 +253,7 @@ static void breakpoint_invalidate(CPUState *cpu, target_ulong pc)
>> * Flush the whole TB cache to force re-translation of such TBs.
>> * This is heavyweight, but we're debugging anyway.
>> */
>> - tb_flush(cpu);
>> + /* tb_flush(cpu); */
>> }
>> #endif
>
> The patch is clearly wrong -- this function is called when a CPU breakpoint
> is added or removed, and we *must* drop generated code which either
> (a) includes code to take the breakpoint exception and now should not
> or (b) doesn't include code to take the breakpoint exception and now should.
> Otherwise we will incorrectly take/not take a breakpoint exception when
> that stale code is executed.
>
> As the comment notes, the assumption is that we won't be adding and
> removing breakpoints except when we're debugging and therefore
> performance is not critical. Windows XP is clearly doing something
> we weren't expecting, so we should ideally have a look at whether
> we can be a bit more efficient about not throwing the whole TB
> cache away.
FWIW this was reported a while back on Launchpad as
https://bugs.launchpad.net/qemu/+bug/1883593 where the performance regression was
traced back to:
commit b55f54bc965607c45b5010a107a792ba333ba654
Author: Max Filippov <jcmvbkbc@gmail.com>
Date: Wed Nov 27 14:06:01 2019 -0800
exec: flush CPU TB cache in breakpoint_invalidate
When a breakpoint is inserted at location for which there's currently no
virtual to physical translation no action is taken on CPU TB cache. If a
TB for that virtual address already exists but is not visible ATM the
breakpoint won't be hit next time an instruction at that address will be
executed.
Flush entire CPU TB cache in breakpoint_invalidate to force
re-translation of all TBs for the breakpoint address.
This change fixes the following scenario:
- linux user application is running
- a breakpoint is inserted from QEMU gdbstub for a user address that is
not currently present in the target CPU TLB
- an instruction at that address is executed, but the external debugger
doesn't get control.
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Message-Id: <20191127220602.10827-2-jcmvbkbc@gmail.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Perhaps Windows XP is constantly executing some kind of breakpoint instruction or
updating some CPU debug registers during boot?
ATB,
Mark.
next prev parent reply other threads:[~2021-06-10 13:25 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-10 12:59 tb_flush() calls causing long Windows XP boot times Programmingkid
2021-06-10 13:14 ` Peter Maydell
2021-06-10 13:24 ` Mark Cave-Ayland [this message]
2021-06-11 11:24 ` Alex Bennée
2021-06-11 15:01 ` Programmingkid
2021-06-11 17:13 ` Paolo Bonzini
2021-06-11 18:22 ` Alex Bennée
2021-06-13 14:03 ` Mark Cave-Ayland
2021-06-14 14:37 ` Alex Bennée
2021-06-15 13:58 ` Programmingkid
2021-06-16 1:58 ` Richard Henderson
2021-06-16 8:59 ` Mark Cave-Ayland
2021-06-16 12:53 ` Alex Bennée
2021-06-16 13:06 ` Peter Maydell
2021-06-16 15:30 ` Alex Bennée
2021-06-16 13:21 ` Alex Bennée
2021-06-16 12:12 ` Programmingkid
2021-06-10 13:38 ` Programmingkid
2021-06-14 22:19 ` no-reply
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