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

[Richard Henderson <rth@twiddle.net>]

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?


r~