Re: [patch 02/38] x86/cpu: Use native_wrmsrl() in load_percpu_segment()

Thomas Gleixner
2022-07-18T08:55:37Z

On Mon, Jul 18 2022 at 08:54, Thomas Gleixner wrote:
> On Mon, Jul 18 2022 at 07:11, Juergen Gross wrote:
>>> - switch_to_new_gdt(cpu);
>>> + switch_to_real_gdt(cpu);
>>
>> ... can't you use the paravirt variant of load_gdt in switch_to_real_gdt() ?
>
> That does not solve the problem of having a disagreement between GDT and
> GS_BASE. Let me dig into this some more.

Bah. The real problem is __loadsegment_simple(gs, 0). After that GS_BASE
is 0. So any per CPU access before setting MSR_GS_BASE back to working
state is going into lala land.

So it's not the GDT. It's the mov 0, %gs which makes stuff go south, but
as %gs is already 0, we can keep the paravirt load_gdt() and use
native_write_msr() and everything should be happy.

Thanks,

tglx