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.
