Re: [PATCH hyperv-next v2] arch/x86: Provide the CPU number in the wakeup AP callback

[Tom Lendacky]

On 4/30/25 14:33, Thomas Gleixner wrote:
> On Wed, Apr 30 2025 at 09:14, Roman Kisel wrote:
>> -static int wakeup_cpu_via_vmgexit(u32 apic_id, unsigned long start_ip)
>> +static int wakeup_cpu_via_vmgexit(u32 apic_id, unsigned long start_ip, int cpu)
> 
> unsigned int cpu please. There are no negative CPU numbers yet :)
> 
>>  {
>>  	struct sev_es_save_area *cur_vmsa, *vmsa;
>>  	struct ghcb_state state;
>> @@ -1187,7 +1187,7 @@ static int wakeup_cpu_via_vmgexit(u32 apic_id, unsigned long start_ip)
>>  	unsigned long flags;
>>  	struct ghcb *ghcb;
>>  	u8 sipi_vector;
>> -	int cpu, ret;
>> +	int ret;
>>  	u64 cr4;
>>  
>>  	/*
>> @@ -1208,15 +1208,6 @@ static int wakeup_cpu_via_vmgexit(u32 apic_id, unsigned long start_ip)
>>  
>>  	/* Override start_ip with known protected guest start IP */
>>  	start_ip = real_mode_header->sev_es_trampoline_start;
>> -
>> -	/* Find the logical CPU for the APIC ID */
>> -	for_each_present_cpu(cpu) {
>> -		if (arch_match_cpu_phys_id(cpu, apic_id))
>> -			break;
>> -	}
>> -	if (cpu >= nr_cpu_ids)
>> -		return -EINVAL;
>> -
> 
> I just looked what arch_match_cpu_phys_id() actually does and I couldn't
> help myself to get a fit of laughter. x86 uses the weak default function
> in drivers/of/cpu.c:
> 
> bool __weak arch_match_cpu_phys_id(int cpu, u64 phys_id)
> {
> 	return (u32)phys_id == cpu;
> }

There is an x86 version of this function in arch/x86/kernel/cpu/topology.c
that overrides the __weak definition and does:

bool arch_match_cpu_phys_id(int cpu, u64 phys_id)
{
	return phys_id == (u64)cpuid_to_apicid[cpu];
}

Thanks,
Tom

> 
> So this loop is the most convoluted way to write:
> 
>        cpu = apic_id;
> 
> which is valid because the to be started CPU must be present, no?
> 
> I'm not opposed against the CPU number argument per se, but the
> justification for it is dubious at best.
> 
> Thanks,
> 
>         tglx