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Re: [Qemu-devel] how do we determine correct guest PC for segfaults in a

From: Richard Henderson
Subject: Re: [Qemu-devel] how do we determine correct guest PC for segfaults in atomic helpers for linux-user mode?
Date: Tue, 14 Nov 2017 10:06:11 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 11/14/2017 09:52 AM, Peter Maydell wrote:
> On 13 November 2017 at 23:31, Richard Henderson <address@hidden> wrote:
>> On 11/13/2017 08:59 PM, Peter Maydell wrote:
>>> Am I misreading my debugger entrails (entirely possible)? How is this
>>> code intended to get the right guest PC for segfaults in these helpers?
>> It looks like we can't.
>> We get it right for system mode, but not linux-user.
> How does it work for system mode?

We have retaddr from GETPC which we pass down through tlb_fill and friends,
which means the correct pc is used for restore state.

What's different about user-mode is that we don't have tlb_fill or equivalent,
and we rely on the pc from the signal handler.  Which leads to the bogusness
that you see.

I've just about got a patch together that uses a TLS variable for retaddr.  It
is the smaller change than setjmp for soft freeze.


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