Re: [Qemu-devel] Call for testing QEMU aarch64-linux-user emulation

["Alex Bennée" <alex.bennee@linaro.org>]

Peter Maydell <peter.maydell@linaro.org> writes:

> On 28 February 2014 14:27, Alexander Graf <agraf@suse.de> wrote:
>> Could we check the instruction at the sognaling pc and check
>> if it's a known syscall instruction? No need to replace glibc
>> wrappers then.
>
> No, because the behaviour we want for "started handling
> syscall in qemu" through to "PC anything up to but not
> including the syscall insn" is "back out and take signal
> then try again", which means we need to be able to unwind
> anything we were doing. If we (effectively) longjmp out of
> the middle of glibc we're liable to leave locked mutexes
> and otherwise mess up glibc internals.

The other option is roll the real PC forward until you know you are at a
point that everything is in a known state - in this case a labelled
syscall instruction. You can achieve this with a host interpreter (which
would be a lot of work to add to QEMU) or maybe achieve the same magic
with ptrace?

If you really want to avoid too much messing about you mask off all your
signals until you really know you can do something about them.

It goes without saying I hope that any serious attempt to fix this needs
a decent set of test cases because the edge cases are numerous.

-- 
Alex Bennée