Re: [RFC] xen/arm64: livepatch: enable attaching callbacks

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 30/06/2026 11:43 am, Ryo Takakura wrote:
>>> diff --git a/xen/arch/arm/arm64/livepatch.c b/xen/arch/arm/arm64/livepatch.c
>>> index e135bd5bf9..b7c9aba94e 100644
>>> --- a/xen/arch/arm/arm64/livepatch.c
>>> +++ b/xen/arch/arm/arm64/livepatch.c
>>> @@ -34,12 +57,87 @@ void arch_livepatch_apply(const struct livepatch_func
>>> *func,
>>>      /* Save old ones. */
>>>      memcpy(state->insn_buffer, func->old_addr, len);
>>>
>>> -    if ( func->new_addr )
>>> +    if ( !func->new_addr )
>>> +    {
>>> +        insn = aarch64_insn_gen_nop();
>>> +    }
>>> +    else if ( func->trampoline_buf )
>>> +    {
>>> +        int rc;
>>> +        uint32_t *trampoline = func->trampoline_buf;
>>> +        uint32_t *tp = trampoline;
>>> +        void *orig_cont_addr = (void *)func->old_addr + len;
>>> +        unsigned int trampoline_code_size = len + 12 * 
>>> ARCH_PATCH_INSN_SIZE;
>>> +        unsigned long trampoline_start = (unsigned long)trampoline &
>>> PAGE_MASK;
>>> +        unsigned long trampoline_end =
>>> +            PAGE_ALIGN((unsigned long)trampoline + trampoline_code_size);
>>> +
>>> +        /*
>>> +         * Make the payload text area writeable while generating
>>> +         * the trampoline instructions.
>>> +         */
>>> +        rc = modify_xen_mappings(trampoline_start, trampoline_end,
>>> +                                 PAGE_HYPERVISOR);
>>> +        if ( rc )
>>> +        {
>>> +            printk(XENLOG_ERR LIVEPATCH
>>> +                   "Failed to make trampoline writable: %d\n", rc);
>>> +            return;
>>> +        }
>> This ought not to be necessary.
>>
>> The trampoline is executable code, so should have space reserved for it
>> in .text of the livepatch.
>>
>> Then, you can identify it simply by references in a new section, without
>> having to have a pointer with a sentinel value (void *)1 in (which MISRA
>> will have a fit at).
> I like this idea as well! I'll try this together with the earlier
> suggestion using __attribute__((no_caller_saved_registers)).

If you reserve space in the function preamble, and use
__attribute__((no_caller_saved_registers)), then you don't need
trampolines at all.

The preamble just needs to turn into `call newfunc` when the callback
function is attached, and then it's regular return will do the right thing.

~Andrew