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Re: [PATCH 1/3] x86: drop NOP_DS_PREFIX
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- Date: Wed, 23 Feb 2022 10:54:53 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Thread-index: AQHYKJ3kfYMYavVLu0efzdcMu0mAtKyg9laA
- Thread-topic: [PATCH 1/3] x86: drop NOP_DS_PREFIX
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 <jbeulich@xxxxxxxx>
Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
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?
~Andrew
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