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Re: [PATCH 2/2] x86: guard synthetic feature and bug enumerators
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 9 Oct 2025 09:51:49 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 09 Oct 2025 07:52:01 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 09.10.2025 09:45, Jan Beulich wrote:
> On 08.10.2025 19:19, Andrew Cooper wrote:
>> On 25/09/2025 11:48 am, Jan Beulich wrote:
>>> --- a/xen/arch/x86/include/asm/pdx.h
>>> +++ b/xen/arch/x86/include/asm/pdx.h
>>> @@ -13,9 +13,9 @@
>>> asm_inline goto ( \
>>> ALTERNATIVE( \
>>> "", \
>>> - "jmp %l0", \
>>> - ALT_NOT(X86_FEATURE_PDX_COMPRESSION)) \
>>> - : : : : label )
>>> + "jmp %l1", \
>>> + [feat]) \
>>> + : : [feat] "i" (ALT_NOT(X86_FEATURE_PDX_COMPRESSION)) : : label )
>>
>> Not a bug in this change, but the pre-existing use of positional labels
>> is something I was expecting not to introduce at all seeing as we
>> started cleanly with named labels.
>>
>> The jmp wants to be:
>>
>> "jmp %l" #label
>>
>> to cope with the fact it's a macro parameter too.
>
> Unrelated change? I can of course do the adjustment in a separate prereq
> patch, but then it would have been nice if you had commented along these
> lines before that code actually had gone in.
>
> That said, isn't it at least bad practice to not expose the label use to
> the compiler? To avoid using positional operands, shouldn't we rather
> name the operand, and then use "jmp %l[whatever_the_name]"? That's a
> change I could see as being justified to do right here, rather than in a
> separate patch.
Hmm, no, labels can't be named; they are their own names. I.e. what I think
we want here is "jmp %l[" #label "]".
Jan
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