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Re: [PATCH] x86/amd: Introduce and use X86_BUG_NULL_SEG
- To: Jan Beulich <jbeulich@xxxxxxxx>, Pu Wen <puwen@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 8 Sep 2021 16:52:27 +0100
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- Cc: Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Wed, 08 Sep 2021 15:52:47 +0000
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On 08/09/2021 16:06, Jan Beulich wrote:
> On 08.09.2021 15:59, Andrew Cooper wrote:
>> AMD/Hygon processors before the Zen2 microarchitecture don't clear the base
>> or
>> limit fields when loading a NULL segment.
>>
>> Express the logic in terms of cpu_bug_null_seg, and adjust the workaround in
>> do_set_segment_base().
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Thanks.
> albeit with a question (perhaps to Pu):
>
>> --- a/xen/arch/x86/cpu/hygon.c
>> +++ b/xen/arch/x86/cpu/hygon.c
>> @@ -40,6 +40,13 @@ static void init_hygon(struct cpuinfo_x86 *c)
>> c->x86 == 0x18)
>> detect_zen2_null_seg_behaviour();
>>
>> + /*
>> + * Hygon CPUs before Zen2 don't clear segment bases/limits when
>> + * loading a NULL selector.
>> + */
>> + if (c == &boot_cpu_data && !cpu_has_nscb)
>> + setup_force_cpu_cap(X86_BUG_NULL_SEG);
> Are there any affected CPUs in reality?
Yes - the Hygon Zen1 uarch CPUs are affected.
>
> And actually (only noticing when seeing the malformed context
> above) there look to be indentation issues in the changes to
> both this file and amd.c.
Ah yes - I'll fix on commit.
~Andrew
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