Re: [Xen-devel] [v3 04/12] x86/fsgsbase/64: Enable FSGSBASE instructions in the helper functions

["Bae, Chang Seok" <chang.seok.bae@xxxxxxxxx>]

> On Oct 24, 2018, at 12:16, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> 
> On Tue, Oct 23, 2018 at 11:43 AM Chang S. Bae <chang.seok.bae@xxxxxxxxx> 
> wrote:
>> void x86_fsbase_write_cpu(unsigned long fsbase)
>> {
>> -       /*
>> -        * Set the selector to 0 as a notion, that the segment base is
>> -        * overwritten, which will be checked for skipping the segment load
>> -        * during context switch.
>> -        */
>> -       loadseg(FS, 0);
>> -       wrmsrl(MSR_FS_BASE, fsbase);
>> +       if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
>> +               wrfsbase(fsbase);
>> +       } else {
>> +               /*
>> +                * Set the selector to 0 as a notion, that the segment base 
>> is
>> +                * overwritten, which will be checked for skipping the 
>> segment load
>> +                * during context switch.
>> +                */
>> +               loadseg(FS, 0);
>> +               wrmsrl(MSR_FS_BASE, fsbase);
>> +       }
>> }
>> 
>> void x86_gsbase_write_cpu_inactive(unsigned long gsbase)
>> {
>> -       /* Set the selector to 0 for the same reason as %fs above. */
>> -       loadseg(GS, 0);
>> -       wrmsrl(MSR_KERNEL_GS_BASE, gsbase);
>> +       if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
>> +               wr_inactive_gsbase(gsbase);
>> +       } else {
>> +               /* Set the selector to 0 for the same reason as %fs above. */
>> +               loadseg(GS, 0);
>> +               wrmsrl(MSR_KERNEL_GS_BASE, gsbase);
> 
> I still don't get what this code is trying to do.  See other email.  I
> think it will straight up crash the kernel on some CPUs, since writing
> 0 to %%gs will zero out the *active* base on some CPUs.
> 

On those CPUs, how the old do_arch_prctl_64() worked?
loadseg(GS, 0) eventually hits the native_load_gs_index entry, where actual
mov …, %gs is wrapped by two SWAPGSes. So, it won’t cause the side effect
of overwriting the *active* base, I think.

> I think that, if you really want some fancy optimization for the
> non-FSGSBASE case, you need to pull that out into the callers of these
> helpers.

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