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Re: [Xen-devel] [PATCH] x86/suspend: Simplify system table handling on resume
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 28 Aug 2019 15:16:53 +0100
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- Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 28 Aug 2019 14:17:16 +0000
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On 28/08/2019 15:10, Jan Beulich wrote:
> On 12.08.2019 20:21, Andrew Cooper wrote:
>> load_TR() is used exclusively in the resume path, but jumps through a lot of
>> unnecessary hoops.
>>
>> As suspend/resume is strictly on CPU0 in idle context, the correct GDT to use
>> is boot_gdt, which means it doesn't need saving on suspend. Similarly, the
>> correct IDT to use can be derived, and the LDT is guaranteed to be NUL.
>>
>> The TR is still correct in the GDT, but needs the busy bit clearing before we
>> can reload it.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> ---
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>> CC: Wei Liu <wl@xxxxxxx>
>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>
>> A slightly different option would be to call load_system_tables() rather than
>> opencoding part of it in restore_rest_processor_state(). However, that is
>> more setup than is necessary. Thoughts?
> This might indeed be better (despite the parts not needed on this
> path), as it would further centralize the handling. Of course only
> as long as there's no (significant) change needed to the function
> in order to be used here.
No change at all. The AP's already do go through load_system_tables()
on S3 resume.
It is slightly suboptimal that we get two identical lgdt's this close
together, but it is the only way of ending up entering C with
__HYPERVISOR_CS.
At some future point, I'll see how easy it is to arrange the trampoline
GDT to be usable with __HYPERVISOR_CS, which would simplify the boot and
resume paths.
~Andrew
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