Re: [PATCH] x86/Xen: swap NX determination and GDT setup on BSP

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.05.2021 09:18, Juergen Gross wrote:
> On 20.05.21 14:08, Jan Beulich wrote:
>> On 20.05.2021 13:57, Juergen Gross wrote:
>>> On 20.05.21 13:42, Jan Beulich wrote:
>>>> xen_setup_gdt(), via xen_load_gdt_boot(), wants to adjust page tables.
>>>> For this to work when NX is not available, x86_configure_nx() needs to
>>>> be called first.
>>>>
>>>> Reported-by: Olaf Hering <olaf@xxxxxxxxx>
>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>
>>> Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
>>
>> Thanks. I guess I forgot
>>
>> Cc: stable@xxxxxxxxxxxxxxx
>>
>> If you agree, can you please add this before pushing to Linus?
> 
> Uh, just had a look why x86_configure_nx() was called after
> xen_setup_gdt().
> 
> Upstream your patch will be fine, but before kernel 5.9 it will
> break running as 32-bit PV guest (see commit 36104cb9012a82e7).

Oh, indeed. That commit then actually introduced the issue here,
and hence a Fixes: tag may be warranted.

> So I will take your patch as is, but for kernels 5.8 and older I
> recommend a different approach by directly setting the NX
> capability after checking the cpuid bit instead of letting that
> do get_cpu_cap().

Right - perhaps the only halfway viable option.

64-bit kernels predating 4f277295e54c may then also need that one,
but perhaps all stable ones already have it because it was tagged
for stable.

Jan