From: Programmingkid <programmingkidx@gmail.com>
To: Peter Maydell <peter.maydell@linaro.org>
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 09:38:44 -0400 [thread overview]
Message-ID: <898FF8C0-4BE3-4EEE-B471-F4A367CEF19E@gmail.com> (raw)
In-Reply-To: <CAFEAcA-YuyZ9kyivh1dL_chxrtvBF=jOf3L59JuroL2U-e+Xsg@mail.gmail.com>
> On Jun 10, 2021, at 9:14 AM, Peter Maydell <peter.maydell@linaro.org> 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.
>
> thanks
> -- PMM
Thank you for the information. I think there may be additional conditions that may need to be considered before calling tb_flush().
next prev parent reply other threads:[~2021-06-10 13:44 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
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 [this message]
2021-06-14 22:19 ` no-reply
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