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

Peter Zijlstra
2022-07-18T09:32:23Z

On Mon, Jul 18, 2022 at 10:55:29AM +0200, Thomas Gleixner wrote:
> 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.

How is the ret from xen_load_gdt() not going to explode?