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Re: [PATCH 18/22] x86/traps: Set MSR_PL0_SSP in load_system_tables()
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 14 Aug 2025 20:37:54 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 14 Aug 2025 19:37:57 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 14/08/2025 4:00 pm, Jan Beulich wrote:
> On 08.08.2025 22:23, Andrew Cooper wrote:
>> FRED and IDT differ by a Supervisor Token on the base of the shstk. This
>> means that the value they load into MSR_PL0_SSP differs by 8.
>>
>> s3_resume() in particular has logic which is otherwise invariant of FRED
>> mode,
>> and must not clobber a FRED MSR_PL0_SSP with an IDT one.
>>
>> This also simplifies the AP path too. Updating reinit_bsp_stack() is
>> deferred
>> until later.
>>
>> No functional change.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> I wonder why this was originally done in assembly in the first place, when
> we aim at reducing tghe assembly code we have.
It took several iterations (and releases) to get the setup of the
supervisor tokens correct.
I can't even recall if we had load_system_tables() working like this
when the first code went it. It may have been following a subsequent
clean-up series.
>
>> --- a/xen/arch/x86/boot/x86_64.S
>> +++ b/xen/arch/x86/boot/x86_64.S
>> @@ -65,17 +65,11 @@ ENTRY(__high_start)
>> or $(PRIMARY_SHSTK_SLOT + 1) * PAGE_SIZE - 8, %rdx
>>
>> /*
>> - * Write a new supervisor token. Doesn't matter on boot, but for S3
>> - * resume this clears the busy bit.
>> + * Write a new Supervisor Token. It doesn't matter the first time a
>> + * CPU boots, but for S3 resume or CPU hot re-add, this clears the
>> + * busy bit.
>> */
>> wrssq %rdx, (%rdx)
>> -
>> - /* Point MSR_PL0_SSP at the token. */
>> - mov $MSR_PL0_SSP, %ecx
>> - mov %edx, %eax
>> - shr $32, %rdx
>> - wrmsr
>> -
>> setssbsy
> This is ending up a little odd: The comment says the write is to clear the
> busy bit, when that's re-set immediately afterwards.
That comment is about the wrssq. I suppose what isn't said is that
setssbsy will fault if not. How about ", so SETSSBSY can set it again" ?
~Andrew
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