Re: [Qemu-devel] [PATCH v2 2/2] target-arm: Fix CPU breakpoint handling

[Peter Maydell]

On 9 October 2015 at 14:59, Sergey Fedorov <address@hidden> wrote:
> On 08.10.2015 21:40, Peter Maydell wrote:
>> Annoying corner case which I don't think we need to handle necessarily:
>> if you set a breakpoint on a 32-bit Thumb instruction which spans a page
>> boundary, and the second page is not present, we will end up taking the
>> page fault when I think we should take the breakpoint. I can't think
>> of a way to get that right, so just commenting that it isn't handled
>> right would do.
>
> Could you please point out the piece of code which will generate the
> page fault? Maybe I will give it a thought :)

When you call arm_ldl_code() and friends, they will end up longjmp()ing
out of the codegen phase if the load faults. This then turns into a
guest-visible fault in the usual way.

To avoid this you'd need to instead call functions which return
a transaction status, but then:
 (a) you need to restructure the translate.c code so it can
 deal with the idea of backing out if the instruction isn't
 actually present
 (b) this still wouldn't work for linux-user mode, where we
 don't have any way to say "do a memory access, but let me know
 if it would fail rather than longjmping"

thanks
-- PMM