Re: [RFT PATCH] x86/hyperv: Use __naked attribute to fix stackless C function

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 26/02/2026 1:29 pm, Ard Biesheuvel wrote:
>
> On Thu, 26 Feb 2026, at 14:24, Andrew Cooper wrote:
>> On 26/02/2026 1:07 pm, Ard Biesheuvel wrote:
>>> On Thu, 26 Feb 2026, at 13:01, Andrew Cooper wrote:
>>>>> @@ -133,49 +150,36 @@ static noinline void hv_crash_clear_kernpt(void)   * available. We restore kernel GDT, and rest of the context, and continue
>>>>>   * to kexec.
>>>>>   */
>>>>> -static asmlinkage void __noreturn hv_crash_c_entry(void) +static void
>>>>> __naked hv_crash_c_entry(void)  {
>>>>> - struct hv_crash_ctxt *ctxt = &hv_crash_ctxt; -  	/* first thing, restore kernel gdt */
>>>>> - native_load_gdt(&ctxt->gdtr); + asm volatile("lgdt %0" : : "m"
>>>>> (hv_crash_ctxt.gdtr));  
>>>>> - asm volatile("movw %%ax, %%ss" : : "a"(ctxt->ss)); - asm
>>>>> volatile("movq %0, %%rsp" : : "m"(ctxt->rsp)); + asm volatile("movw
>>>>> %%ax, %%ss" : : "a"(hv_crash_ctxt.ss)); + asm volatile("movq %0,
>>>>> %%rsp" : : "m"(hv_crash_ctxt.rsp));
>>>> I know this is pre-existing, but the asm here is poor.
>>>>
>>>> All segment registers loads can have a memory operand, rather than
>>>> forcing through %eax, which in turn reduces the setup logic the compiler
>>>> needs to emit.
>>>>
>>>> Something like this:
>>>>
>>>>     "movl %0, %%ss" : : "m"(hv_crash_ctxt.ss)
>>>>
>>>> ought to do.
>>>>
>>> 'movw' seems to work, yes.
>> movw works, but is sub-optimal.
>>
> Can you give an asm example where movl with a segment register is accepted by the assembler? I only managed that with movw, hence my comment.

Oh lovely, that looks like a binutils bug, but I bet it comes from not
realising that `mov sreg` is different to the more general mov forms.

Using no suffix will emit the optimal instruction without a warning.

https://godbolt.org/z/GYKs31Gqn

~Andrew