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Re: [PATCH v2] x86/entry: Partially revert IST-exit checks
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 19 Sep 2023 17:19:23 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 19 Sep 2023 15:19:33 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 19.09.2023 17:01, Andrew Cooper wrote:
> The patch adding check_ist_exit() neglected to consider reset_stack_and_jump()
> leaving C and entering one of the Xen exit paths. The value in %r12 is stale,
> and depending on compiler decisions may not be 0.
And it may also not be zero that we would be looking for. I think this
wants expressing differently. The value in %r12 simply doesn't survive,
and this has at best little to do with compiler decisions.
> This shows up in Gitlab CI for the Clang build:
>
> https://gitlab.com/xen-project/people/andyhhp/xen/-/jobs/5112783827
>
> and in OSSTest for GCC 8:
>
>
> http://logs.test-lab.xenproject.org/osstest/logs/183045/test-amd64-amd64-xl-qemuu-debianhvm-amd64/serial-pinot0.log
>
> There's no straightforward way to reconstruct the IST-exit-ness on the
> exit-to-guest path after a context switch. For now, we only need IST-exit on
> the return-to-Xen path.
>
> Fixes: 21bdc25b05a0 ("x86/entry: Track the IST-ness of an entry for the exit
> paths")
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Code change itself:
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Jan
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