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Re: [PATCH v5 1/4] x86: record SSP at non-guest entry points
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 10 Nov 2026 09:26:53 +0100
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- Cc: Teddy Astie <teddy.astie@xxxxxxxxxx>, Roger Pau Monné <roger@xxxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 10 Nov 2026 08:27:03 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 09.11.2026 19:30, Andrew Cooper wrote:
> On 09/11/2026 1:08 pm, Jan Beulich wrote:
>> We will want to use that value for call trace generation, and likely
>> also to eliminate the somewhat fragile shadow stack searching done in
>> fixup_exception_return(). For those purposes, guest-only entry points do
>> not need to record that value.
>>
>> To keep the saving code simple, record our own SSP that corresponds to
>> an exception frame, pointing to the top of the shadow stack counterpart
>> of what the CPU has saved on the regular stack. Consuming code can then
>> work its way from there.
>>
>> In SAVE_ALL / RESTORE_ALL simply drop the use of UREGS_r15. We want the
>> full size in all cases, so what exactly the top-of-stack field is going
>> to be (whose UREGS_* is 0 anyway) doesn't matter this much there. This
>> way we don't need to distinguish between XEN_SHSTK=y and XEN_SHSTK=n.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> The code is now fine, but the commit message is quite stale.
>
> You need to drop "non-guest" in at least two places, and change
> SAVE_ALL/RESTORE_ALL. In fact that whole final paragraph can go.
Oh, indeed. Dropped / adjusted.
> With the commit message suitably adjusted, Reviewed-by: Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx>
Thanks.
>> --- a/xen/arch/x86/include/asm/asm_defns.h
>> +++ b/xen/arch/x86/include/asm/asm_defns.h
>> @@ -259,12 +259,19 @@ static always_inline void stac(void)
>> xor %r14d, %r14d
>> push %r15
>> xor %r15d, %r15d
>> +#ifdef CONFIG_XEN_SHSTK
>> + rdsspq %rcx
>> + push %rcx
>> +#endif
>
> It might be worth having /* Zero (from above) if CET-SS is not enabled
> */ on the push here to make it clear what value is stored.
Sure, added.
Jan
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