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Re: [PATCH v1] xen: Work around Clang-IAS macro expansion bug
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 21 Feb 2023 13:10:48 +0000
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 21 Feb 2023 13:11:31 +0000
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On 21/02/2023 12:46 pm, Jan Beulich wrote:
> On 21.02.2023 13:26, Andrew Cooper wrote:
>> On 21/02/2023 10:31 am, Jan Beulich wrote:
>>> On 17.02.2023 13:22, Andrew Cooper wrote:
>>>> https://github.com/llvm/llvm-project/issues/60792
>>>>
>>>> It turns out that Clang-IAS does not expand \@ uniquely in a translaition
>>>> unit, and the XSA-426 change tickles this bug:
>>>>
>>>> <instantiation>:4:1: error: invalid symbol redefinition
>>>> .L1_fill_rsb_loop:
>>>> ^
>>>> make[3]: *** [Rules.mk:247: arch/x86/acpi/cpu_idle.o] Error 1
>>>>
>>>> Extend DO_OVERWRITE_RSB with an optional parameter so C callers can mux %=
>>>> in
>>>> too, which Clang does seem to expand properly.
>>>>
>>>> Fixes: 63305e5392ec ("x86/spec-ctrl: Mitigate Cross-Thread Return Address
>>>> Predictions")
>>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>> ---
>>>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>>>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>>> CC: Wei Liu <wl@xxxxxxx>
>>>>
>>>> v1 (vs RFC):
>>>> * Rename \foo to \x, reorder WRT \@ to avoid needing \() too.
>> Sadly, this is not quite correct. There needs to be a non-numeric
>> character separating \@ and \x or the concatenation of the two can end
>> up non-unique.
> How that if \@ is always 1?
It isn't always 1.
Global (file scope) have \@ expand properly from 0 to N.
Function scope have \@ expand properly from 0 to N in a single asm()
statement.
The 1 here is actually because mknops took the 0 and didn't use it. If
instead we had something like:
asm ("DO_OVERWRITE_RSB"); // \@ = 0
asm ("mkops 2;"
"DO_OVERWRITE_RSB"); // \@ = 1
then it would assemble fine, but the way we build alternatives in asm()
statements reliably causes the alternative block to consume \@=1.
>> So I think we need the \().
> Or put one at the start of the label and the other at the end?
We already played that trick with \n for the .irp, which suffers
similarly with concatenation.
~Andrew
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