Re: [Xen-devel] [PATCH v3 16/25] x86emul: support SWAPGS

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.02.18 at 14:41, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 07/12/17 14:11, Jan Beulich wrote:
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> v3: New.
>>
>> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
>> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
>> @@ -5047,6 +5047,24 @@ x86_emulate(
>>                  goto done;
>>              break;
>>  
>> +        case 0xf8: /* swapgs */
>> +            generate_exception_if(!mode_64bit(), EXC_UD);
>> +            generate_exception_if(!mode_ring0(), EXC_GP, 0);
>> +            fail_if(!ops->read_segment || !ops->read_msr ||
>> +                    !ops->write_segment || !ops->write_msr);
>> +            if ( (rc = ops->read_segment(x86_seg_gs, &sreg,
>> +                                         ctxt)) != X86EMUL_OKAY ||
>> +                 (rc = ops->read_msr(MSR_SHADOW_GS_BASE, &msr_val,
>> +                                     ctxt)) != X86EMUL_OKAY ||
>> +                 (rc = ops->write_msr(MSR_SHADOW_GS_BASE, sreg.base,
>> +                                      ctxt)) != X86EMUL_OKAY )
> 
> We need to unwind this write in the case of write_segment failing, or
> when the instruction restarts, state will be corrupt.

We don't do similar restoring anywhere else iirc, so I'm not sure I
want to start doing so here. Multi-element updates really need to
be converted to go through a staging layer, where the checks
are done right away, but the commit happens only at the end.
One of the reasons I decided against doing what you suggest
(indeed I had considered that) is that this other write may then
be failing, too. Let me know.

Jan

>> +                goto done;
>> +            sreg.base = msr_val;
>> +            if ( (rc = ops->write_segment(x86_seg_gs, &sreg,
>> +                                          ctxt)) != X86EMUL_OKAY )
>> +                goto done;
>> +            break;
>> +
>>          case 0xf9: /* rdtscp */
>>              fail_if(ops->read_msr == NULL);
>>              if ( (rc = ops->read_msr(MSR_TSC_AUX,
>>
>>
>>




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