Re: [Qemu-devel] [PATCH] target-*: Advance pc after recognizing a breakpoint

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

On 19.10.2015 20:04, Richard Henderson wrote:
> On 10/19/2015 01:04 AM, Sergey Fedorov wrote:
>> On 19.10.2015 01:46, Richard Henderson wrote:
>>> On 10/16/2015 04:08 AM, Sergey Fedorov wrote:
>>>> On 16.10.2015 04:14, Richard Henderson wrote:
>>>>> On 10/16/2015 03:36 AM, Peter Maydell wrote:
>>>>>> On 14 October 2015 at 22:02, Richard Henderson <rth@twiddle.net>
>>>>>> wrote:
>>>>>>> On 10/15/2015 06:34 AM, Peter Maydell wrote:
>>>>>>>>
>>>>>>>> This is still the same cryptic comment we have in the
>>>>>>>> targets which do do this. Can we have something
>>>>>>>> that is a bit more explanatory about what is going on and
>>>>>>>> why we need to do this, please?
>>>>>>>
>>>>>>>
>>>>>>> Suggestions?
>>>>>>
>>>>>> ...well, I don't entirely understand the problem it's
>>>>>> fixing, which is why I'm asking for a better comment :-)
>>>>>
>>>>> Heh.  Fair enough.  How about
>>>>>
>>>>>     /* The address covered by the breakpoint must be included in
>>>>>        [tb->pc, tb->pc + tb->size) in order to for it to be
>>>>>        properly cleared -- thus we increment the PC here so that
>>>>>        the logic setting tb->size below does the right thing.  */
>>>>>
>>>>> There are two edge cases that cause the problem with clearing that
>>>>> could be described, but I think that the comment becomes too
>>>>> bulky, as
>>>>> well as confuses the situation for someone cutting-and-pasting the
>>>>> logic to a new port.
>>>>
>>>> Maybe we could rather fix that condition in
>>>> tb_invalidate_phys_page_range()? It seems weird that it can't handle a
>>>> zero-sized TB.
>>>
>>> We also need to be able to handle a TB which crosses a page.  E.g. the
>>> breakpoint is at the page boundary, and we fall through into it from
>>> the top. This will be true on e.g. x86.  This is not simply true for
>>> breakpoint insertion/removal, but also page invalidation.
>>>
>>> The same fix, adding a byte to the size, handles this as well.
>>
>> It's clear except that instructions crossing a page boundary can be
>> different in size. AFAIK, x86 instructions can be up to 15-byte long.
>> What if only the very last byte of instruction crosses a page boundary?
>
> Then only the last byte crosses?  What's your point?

My point is if "adding a byte to the size" handles such case as well.

Best,
Sergey