Re: [Xen-devel] [PATCH] x86/build: Use new .nops directive when available

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.08.18 at 12:42, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 16/08/18 10:55, Roger Pau Monné wrote:
>> On Wed, Aug 15, 2018 at 06:57:38PM +0100, Andrew Cooper wrote:
>>> @@ -112,6 +125,11 @@ static void __init arch_init_ideal_nops(void)
>>>              ideal_nops = k8_nops;
>>>          break;
>>>      }
>>> +
>>> +#ifdef HAVE_AS_NOP_DIRECTIVE
>>> +    if ( memcmp(ideal_nops[ASM_NOP_MAX], toolchain_nops, ASM_NOP_MAX) == 0 
>>> )
>>> +        toolchain_nops_are_ideal = true;
>>> +#endif
>> You are only comparing that the biggest nop instruction (9 bytes
>> AFAICT) generated by the assembler is what Xen believes to be the more
>> optimized version. What about shorter nops?
> 
> They are all variations on a theme.
> 
> For P6 nops, its the 0f 1f root which is important, which takes a modrm
> byte.  Traditionally, its always encoded with eax and uses redundant
> memory encodings for longer instructions.
> 
> I can't think of any way of detecting if the optimised nops if the
> toolchain starts using alternative registers in the encoding, but I
> expect this case won't happen in practice.

It's not just the register encoding, but also the maximum single-insn
length that gets generated. Recall that until not very long ago we
had up to 8-byte NOP insns only? The view on the mod (as in ModRM)
usage may vary over time, as may the view on which or how many
prefixes are reasonable to have.

Jan


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