Re: [PATCH 2/6] x86/entry_64: Add VERW just before userspace transition

[Dave Hansen]

On 10/23/23 11:22, Josh Poimboeuf wrote:
> On Fri, Oct 20, 2023 at 01:45:03PM -0700, Pawan Gupta wrote:
>> +	/* Mitigate CPU data sampling attacks .e.g. MDS */
>> +	USER_CLEAR_CPU_BUFFERS
>> +
>>  	jmp	.Lnative_iret
>>  
>>  
>> @@ -774,6 +780,9 @@ native_irq_return_ldt:
>>  	 */
>>  	popq	%rax				/* Restore user RAX */
>>  
>> +	/* Mitigate CPU data sampling attacks .e.g. MDS */
>> +	USER_CLEAR_CPU_BUFFERS
>> +
> 
> I'm thinking the comments add unnecessary noise here.  The macro name is
> self-documenting enough.
> 
> The detail about what mitigations are being done can go above the macro
> definition itself, which the reader can refer to if they want more
> detail about what the macro is doing and why.
> 
> Speaking of the macro name, I think just "CLEAR_CPU_BUFFERS" is
> sufficient.  The "USER_" prefix makes it harder to read IMO.

Yes, please.  The "USER_" prefix should be reserved for things that are
uniquely for the unambiguous return-to-userspace paths.