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Re: [PATCH] x86: Fix check_ist_exit() assertions
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 19 Sep 2023 13:46:03 +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 11:46:20 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 19.09.2023 12:35, 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.
>
> 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
>
> The justification for ensuring ist_exit is accurate in the exit paths still
> stands, so zero %r12 in reset_stack_and_jump() to indicate a non-IST exit.
I did think of this as an option, but I don't think this covers all cases.
If we take #DB while in a PV guest, that'll be an IST entry. Assume further
that we re-schedule before re-entering the guest. Upon the vCPU being
scheduled back in we'll have %r12 clear with an on-stack indication of
having taken an IST guest exit.
Jan
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