Re: [PATCH 2/6] x86/sev: add support for enabling RMPOPT

[Kalra, Ashish]

On 2/18/2026 8:59 AM, Dave Hansen wrote:
> On 2/17/26 19:08, K Prateek Nayak wrote:
>> Hello Dave,
>>
>> On 2/18/2026 3:36 AM, Dave Hansen wrote:
>>>> +/*
>>>> + * Build a cpumask of online primary threads, accounting for primary threads
>>>> + * that have been offlined while their secondary threads are still online.
>>>> + */
>>>> +static void get_cpumask_of_primary_threads(cpumask_var_t cpulist)
>>>> +{
>>>> +	cpumask_t cpus;
>>>> +	int cpu;
>>>> +
>>>> +	cpumask_copy(&cpus, cpu_online_mask);
>>>> +	for_each_cpu(cpu, &cpus) {
>>>> +		cpumask_set_cpu(cpu, cpulist);
>>>> +		cpumask_andnot(&cpus, &cpus, cpu_smt_mask(cpu));
>>>> +	}
>>>> +}
>>>
>>> Don't we have a primary thread mask already? I thought we did.
>>
>> If you are referring to cpu_primary_thread_mask(), the CPUs are set on it
>> based on the LSB of APICID, specifically:
>>
>>     !(apicid & (__max_threads_per_core - 1))
>>
>> It can so happen, the primary thread ((apicid & 1) == 0) of the core is
>> offline while the secondary thread ((apicid & 1) == 1) is online but the
>> traversal of (cpu_primary_thread_mask() & cpu_online_mask()) will simply
>> skip these cores.
>>
>> Is there an equivalent mask that sets the first online CPU of each core?
> 
> No I don't think we have that sitting around.
> 
> But, stepping back, why is this even necessary? Is it just saving a few
> IPIs in the super rare case that someone has offlined the primary thread
> but not a secondary one?
> 
> Why bother?

Because, setting RMPOPT_BASE MSR (which is a per-core MSR) and RMPOPT instruction
need to be issued on only one thread per core. If the primary thread is offlined
and secondary thread is not considered, we will miss/skip setting either the
RMPOPT_BASE MSR or not issuing the RMPOPT instruction for that physical CPU, which means
no RMP optimizations enabled for that physical CPU.

Thanks,
Ashish