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Re: [RFC 5/5] xen/x86: livepatch: enable attaching callbacks
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Ryo Takakura <takakura@xxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 26 Oct 2026 12:56:56 +0100
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- Cc: roger@xxxxxxxxxxxxxx, ross.lagerwall@xxxxxxxxxx, sstabellini@xxxxxxxxxx, julien@xxxxxxx, bertrand.marquis@xxxxxxx, michal.orzel@xxxxxxx, Volodymyr_Babchuk@xxxxxxxx, anthony.perard@xxxxxxxxxx, teddy.astie@xxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Mon, 26 Oct 2026 11:57:08 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 26.10.2026 12:42, Andrew Cooper wrote:
> On 23/10/2026 5:58 am, Ryo Takakura wrote:
>> +FUNC(xen_livepatch_trace_caller)
>> + pushq %rbp
>> + movq %rsp, %rbp
>> + pushfq
>> +
>> + /* Preserve general-purpose registers. */
>> + pushq %rax
>> + pushq %rcx
>> + pushq %rdx
>> + pushq %rsi
>> + pushq %rdi
>> + pushq %r8
>> + pushq %r9
>> + pushq %r10
>> + pushq %r11
>> +
>> + /*
>> + * xen_livepatch_trace_dispatcher(ip, parent_ip)
>> + *
>> + * ip: Address of the traced function.
>> + * parent_ip: Address where the traced function was called.
>> + */
>> + movq 8(%rbp), %rdi
>> + subq $5, %rdi
>> + movq 16(%rbp), %rsi
>> + call xen_livepatch_trace_dispatcher
>> +
>> + /* Restore the saved state. */
>> + popq %r11
>> + popq %r10
>> + popq %r9
>> + popq %r8
>> + popq %rdi
>> + popq %rsi
>> + popq %rdx
>> + popq %rcx
>> + popq %rax
>> + popfq
>> + popq %rbp
>> + ret
>> +END(xen_livepatch_trace_caller)
>
> Just in case there's a problem doing this all in C...
>
> No need for pushf or popf at all. Flags are undefined at function
> boundaries.
>
> Please drop the q suffixes; they're all unnecessary here.
>
> The ret needs to be RET for speculative mitigations. (It's emitted as
> JMP which will be turned back into a RET on safe system.)
Just for completeness: "will" as in "will eventually"; see my still
pending "x86: re-inline RET used in assembly code" (with me doing C side
work still pending feedback for that initial, smaller change).
Jan
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