Re: [Qemu-devel] [PATCH] target-*: Get rid of "PC advancement" trick

[Sergey Fedorov <serge.fdrv@gmail.com>]

On 12/12/15 02:39, Richard Henderson wrote:
> On 12/10/2015 10:47 AM, Sergey Fedorov wrote:
>> The "PC advancement" trick was used just after recognizing that a
>> breakpoint exception was going to be generated. This trick has had two
>> points:
>>   1. Guarantee that tb->size isn't zero: there are many places where 
>> it's
>>      expected to be non-zero. In fact, that is even stated in the 
>> comment
>>      for this field.
>>   2. Try to satisfy disassembler's check for instruction length. To this
>>      end, PC advancement was done for estimated instruction length, but
>>      actually, didn't work properly in variable-instruction-length 
>> cases.
>>
>> Substitute this trick with checking for TB size at the end of
>> translation. If we get an empty TB then just set tb->size to 1 and skip
>> disassembling. Setting tb->size to 1 is enough to get correct behaviour,
>> whereas an empty TB doesn't obviously need to be disassembled.
>
> This doesn't help when the TB already has instructions, the TB would 
> ordinarily cross a page boundary, and the breakpoint is at the page 
> boundary.

I see your point. But I am wondering why most architectures stop 
translating on a page boundary whereas i386 and m86k don't. There are 
some comments which say that's to ensure instruction fetch aborts occur 
at the right place. Isn't it necessary for all architectures?

At least for those architectures which do stop translating on a page 
boundary, I think this patch is applicable. Certainly, it would be 
better to have a single solution for all architectures.

Thanks,
Sergey