Re: [PATCH 09/22] x86/traps: Move load_system_tables() into traps-setup.c

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.08.2025 13:36, Andrew Cooper wrote:
> On 12/08/2025 10:43 am, Nicola Vetrini wrote:
>> On 2025-08-08 22:23, Andrew Cooper wrote:
>>> diff --git a/xen/arch/x86/traps-setup.c b/xen/arch/x86/traps-setup.c
>>> index 8ca379c9e4cb..13b8fcf0ba51 100644
>>> --- a/xen/arch/x86/traps-setup.c
>>> +++ b/xen/arch/x86/traps-setup.c
>>> @@ -19,6 +20,124 @@ boolean_param("ler", opt_ler);
>>>
>>>  void nocall entry_PF(void);
>>>
>>> +/*
>>> + * Sets up system tables and descriptors for IDT devliery.
>>> + *
>>> + * - Sets up TSS with stack pointers, including ISTs
>>> + * - Inserts TSS selector into regular and compat GDTs
>>> + * - Loads GDT, IDT, TR then null LDT
>>> + * - Sets up IST references in the IDT
>>> + */
>>> +static void load_system_tables(void)
>>> +{
>>> +    unsigned int i, cpu = smp_processor_id();
>>> +    unsigned long stack_bottom = get_stack_bottom(),
>>> +        stack_top = stack_bottom & ~(STACK_SIZE - 1);
>>> +    /*
>>> +     * NB: define tss_page as a local variable because clang 3.5
>>> doesn't
>>> +     * support using ARRAY_SIZE against per-cpu variables.
>>> +     */
>>> +    struct tss_page *tss_page = &this_cpu(tss_page);
>>> +    idt_entry_t *idt = this_cpu(idt);
>>> +
>>
>> Given the clang baseline this might not be needed anymore?
> 
> Hmm.  While true, looking at 51461114e26, the code is definitely better
> written with the tss_page variable and we wouldn't want to go back to
> the old form.
> 
> I think that I'll simply drop the comment.
> 
> ~Andrew
> 
> P.S.
> 
> Generally speaking, because of the RELOC_HIDE() in this_cpu(), any time
> you ever want two accesses to a variable, it's better (code gen wise) to
> construct a pointer to it and use the point multiple times.
> 
> I don't understand why there's a RELOC_HIDE() in this_cpu().  The
> justification doesn't make sense, but I've not had time to explore what
> happens if we take it out.

There's no justification in xen/percpu.h?

My understanding is that we simply may not expose any accesses to per_cpu_*
variables directly to the compiler, or there's a risk that it might access
the "master" variable (i.e. CPU0's on at least x86).

Jan