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


From: Sergey Fedorov
Subject: Re: [Qemu-arm] [Qemu-devel] [PATCH] target-*: Get rid of "PC advancement" trick
Date: Sat, 12 Dec 2015 23:02:00 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.4.0

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



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