Re: tb_flush() calls causing long Windows XP boot times

[Programmingkid]

> 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().