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Re: [PATCH] x86/elf: Improve code generation in elf_core_save_regs()
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 11 Mar 2025 15:40:13 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 11 Mar 2025 14:40:18 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 25.02.2025 23:45, Andrew Cooper wrote:
> A CALL with 0 displacement is handled specially, and is why this logic
> functions even with CET Shadow Stacks active. Nevertheless a rip-relative LEA
> is the more normal way of doing this in 64bit code.
>
> The retrieval of flags modifies the stack pointer so needs to state a
> dependency on the stack pointer. Despite it's name, ASM_CALL_CONSTRAINT is
> the way to do this.
>
> read_sreg() forces the answer through a register, causing code generation of
> the form:
>
> mov %gs, %eax
> mov %eax, %eax
> mov %rax, 0x140(%rsi)
>
> Encode the reads directly with a memory operand. This results in a 16bit
> store instead of an 64bit store, but the backing memory is zeroed.
>
> While cleaning this up, drop one piece of trailing whitespace.
>
> No functional change.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
With the plan to ultimately drop read_sreg() altogether:
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Jan
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