On 23.02.2022 11:54, Andrew Cooper wrote:
> On 23/02/2022 10:12, Jan Beulich wrote:
>> This wasn't really necessary to introduce: The binutils change
>> permitting use of standalone "ds" (and "cs") in 64-bit code predates
>> the minimum binutils version we support.
>>
>> Signed-off-by: Jan Beulich <jbeul...@suse.com>
>
> Acked-by: Andrew Cooper <andrew.coop...@citrix.com>
Thanks.
> I was never a fan of NOP_DS_PREFIX. Far too verbose for what it's doing.
>
>> ---
>> In fact we could patch _just_ the opcode prefix in flush_area_local().
>>
>> --- a/xen/arch/x86/flushtlb.c
>> +++ b/xen/arch/x86/flushtlb.c
>> @@ -247,8 +247,7 @@ unsigned int flush_area_local(const void
>> {
>> alternative("", "sfence", X86_FEATURE_CLFLUSHOPT);
>> for ( i = 0; i < sz; i += c->x86_clflush_size )
>> - alternative_input(".byte " __stringify(NOP_DS_PREFIX) ";"
>> - " clflush %0",
>> + alternative_input("ds; clflush %0",
>
> Binutils appears to be happy with "ds clflush", i.e. treating it like a
> proper prefix on the instruction. Drop the semicolon at the same time?
I'd rather not. A clever assembler may eliminate the prefix as redundant
when the base register isn't stack or frame pointer. In 64-bit mode an
assembler might even decide to eliminate all non-standalone segment
overrides using the pre-386 segment registers.
Jan
