Re: [Xen-devel] [PATCH v5 02/10] x86emul: support MOVDIRI insn

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.03.2020 21:58, Andrew Cooper wrote:
> On 24/03/2020 12:29, Jan Beulich wrote:
>> Note that SDM revision 070 doesn't specify exception behavior for
>> ModRM.mod == 0b11; assuming #UD here.
> 
> Didn't I confirm this behaviour for you last time around?

Iirc you did, but the SDM still hasn't changed. Do you have a
suggestion on alternative wording.

>> @@ -10075,6 +10079,14 @@ x86_emulate(
>>                              : "0" ((uint32_t)src.val), "rm" (_regs.edx) );
>>          break;
>>  
>> +    case X86EMUL_OPC(0x0f38, 0xf9): /* movdiri mem,r */
>> +        vcpu_must_have(movdiri);
>> +        generate_exception_if(dst.type != OP_MEM, EXC_UD);
>> +        /* Ignore the non-temporal behavior for now. */
>> +        dst.val = src.val;
>> +        sfence = true;
> 
> Looking again at the SDM, I'm not entirely sure this is good enough.
> 
> Even on top of WB/WP mappings, it needs to have WC properties, knock
> aliasing lines out of the cache, and ending up as a bus transaction.
> 
> Also, I'm not convinced the current chunking algorithm for qemu which
> repeatedly subdivides down to 1, is compatible with the misaligned
> behaviour described, guaranteeing a split of two.

Taking care of these two will be a significant amount of (re-)work of
the HVM emulation layer. I'll see if I can come up with time and ideas
on how to do this.

Jan