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Re: [PATCH v3 14/22] x86/boot: choose AP stack based on APIC ID
- To: Sergii Dmytruk <sergii.dmytruk@xxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 30 Jun 2026 16:23:36 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, trenchboot-devel@xxxxxxxxxxxxxxxx
- Delivery-date: Tue, 30 Jun 2026 14:23:44 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 30.06.2026 16:18, Sergii Dmytruk wrote:
> On Tue, Jun 30, 2026 at 11:37:59AM +0200, Jan Beulich wrote:
>> On 30.06.2026 11:31, Sergii Dmytruk wrote:
>>> On Tue, Jun 30, 2026 at 09:17:59AM +0200, Jan Beulich wrote:
>>>> On 30.06.2026 00:18, Sergii Dmytruk wrote:
>>>>> On Thu, Jan 22, 2026 at 04:52:36PM +0100, Jan Beulich wrote:
>>>>>>> --- a/xen/arch/x86/boot/x86_64.S
>>>>>>> +++ b/xen/arch/x86/boot/x86_64.S
>>>>>>> @@ -15,7 +15,33 @@ ENTRY(__high_start)
>>>>>>> mov $XEN_MINIMAL_CR4,%rcx
>>>>>>> mov %rcx,%cr4
>>>>>>>
>>>>>>> - mov stack_start(%rip),%rsp
>>>>>>> + test %ebx,%ebx
>>>>>>> + cmovz stack_start(%rip), %rsp
>>>>>>> + jz .L_stack_set
>>>>>>> +
>>>>>>> + /* APs only: get stack base from APIC ID saved in %esp. */
>>>>>>> + mov $-1, %rax
>>>>>>
>>>>>> Here and below 32-bit insn would do fine. However, ...
>>>>>
>>>>> Are all addresses guaraneed to be below 4 GiB?
>>>>
>>>> You use %rax as the loop induction variable, so there isn't any address
>>>> in it? Plus you already use ...
>>>>
>>>>>>> + lea x86_cpu_to_apicid(%rip), %rcx
>>>>>>> +1:
>>>>>>> + add $1, %rax
>>>>>>> + cmp $NR_CPUS, %eax
>>>>
>>>> ... as 32-bit insn here anyway.
>>>
>>> But %rcx contains an address and it is added to %rax for `cmp` (quoted
>>> below).
>>
>> Well, to use SCAS you will need to change how you use registers. (IOW
>> first you need to determine whether SCAS actually can be used here,
>> and then - if it can - you will need to arrange register use to fit it.)
>
> It can be used, but I don't see any compelling reason to do it. The
> code doesn't get any shorter and has to recover index from a count.
Okay - if the code doesn't get shorter / easier to follow, then that's a
fair argument.
Jan
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