Re: [Xen-devel] [PATCH 7/7] x86emul: support SYSRET

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.03.2020 11:00, Andrew Cooper wrote:
> On 24/03/2020 16:29, Jan Beulich wrote:
>> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
>> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
>> @@ -5975,6 +5975,60 @@ x86_emulate(
>>              goto done;
>>          break;
>>  
>> +    case X86EMUL_OPC(0x0f, 0x07): /* sysret */
>> +        vcpu_must_have(syscall);
>> +        /* Inject #UD if syscall/sysret are disabled. */
>> +        fail_if(!ops->read_msr);
>> +        if ( (rc = ops->read_msr(MSR_EFER, &msr_val, ctxt)) != X86EMUL_OKAY 
>> )
>> +            goto done;
>> +        generate_exception_if((msr_val & EFER_SCE) == 0, EXC_UD);
> 
> (as with the SYSCALL side), no need for the vcpu_must_have(syscall) as
> well as this check.

Upon re-reading I'm now confused - are you suggesting to also drop
the EFER.SCE check? That's not what you said in reply to 6/7. If
so, what's your thinking behind saying so? If I'm to guess, this
may go along the lines of you suggesting to drop the explicit CPUID
checks from SYSENTER/SYSEXIT as well, but I'm not seeing there
either why you would think this way (albeit there it's also a
little vague what exact changes you're thinking of at the MSR
handling side).

Jan