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Re: [PATCH v2 2/7] tests/x86: Introduce a userspace test harness for x86_decode_lite()
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 4 Oct 2024 09:09:55 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Fri, 04 Oct 2024 07:10:04 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 02.10.2024 17:38, Andrew Cooper wrote:
> On 02/10/2024 4:27 pm, Andrew Cooper wrote:
>> Despite claiming full APX support in the 2.43 release, binutils trunk doesn't
>> tolerate JMPABS at all. Clang-IAS like it but only when encoded as an
>> immediate, despite the fact the operand should be a moffset and encoded
>> without a $ prefix. https://godbolt.org/z/P4Ph3svha
>
> Actually, no. Despite overlaying MOV moffs/rax, to share decode with
> another instruction that has a forced 8-byte immedate operation, it's
> used as a proper immediate, not a memory address.
>
> So on further consideration, the $ prefix is correct.
This one's actually pretty ambiguous. Other direct branches don't use $, i.e.
consider the operand a "displacement", not an "immediate". Except for far
direct branches, which have (two) immediate operands. Originally coming from
the Intel syntax world, where this distinction doesn't even exist, I question
that arrangement. Imo far direct branches should have a non-immediate and an
immediate operand (the latter being the segment / selector). That would also
properly distinguish which one's which. Yet that ship has sailed; we could
add support for the alternative forms, but we can't drop support for the 2-
immediate variants.
I'd prefer to not repeat the mistake for JMPABS: The destination generally
is a symbol, and hence more natural to use without $. In the end it may be
prudent to simply permit both forms.
In any event I've added to my gas TODO list the need for JMPABS support.
Jan
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