Re: [PATCH v10 2/6] x86/sev: Initialize RMPOPT configuration MSRs

[K Prateek Nayak]

Hello Ashish,

On 7/23/2026 6:54 PM, Kalra, Ashish wrote:
> 
> On 7/23/2026 1:50 AM, Kalra, Ashish wrote:
>> Hello Boris,
>>
>> On 7/23/2026 12:14 AM, Borislav Petkov wrote:
>>> On Thu, Jul 23, 2026 at 10:23:56AM +0530, K Prateek Nayak wrote:
>>>> The offline cores will remain offline since hotplug is disabled. RMPOPT
>>>> is simply a performance optimization for RMP checks and leaving the
>>>> offline cores (that will never exit idle) unoptimized should be
>>>> acceptable.
>>>
>>> What happens if you boot with a subset of cores, the boot flow enables RMPOPT
>>> and then you online the rest?
>>>
>>> Have we tried that?
>>>
>>
>>  It can't happen here. snp_prepare() does the following, in order:
>>
>>   1. cpu_hotplug_disable() — freeze the CPU set.
>>   2. fail SNP init (-EOPNOTSUPP) unless every present CPU is online (cpumask_equal(cpu_online_mask, cpu_present_mask).
>>   3. enable SNP.
>>
>>   snp_setup_rmpopt() runs only after snp_prepare() succeeds, so all present CPUs are online and stay online (hotplug is
>>   disabled). No core's primary thread can be offline at this point.
>>
>>   (The kexec path is the one exception — snp_prepare() returns early there — which I'm handling in the other thread.)
>>
>>   Thanks,
>>   Ashish
>>
> 
> Additionally for kexec considerations: on RMPOPT-capable platforms kexec clears SnpEn (full SNP_SHUTDOWN_EX, x86_snp_shutdown=1),
> so snp_prepare() re-runs its cpumask_equal() all-online check on the kexec boot exactly as on a normal boot.
> 
> So no kexec-specific case where RMPOPT gets set up with a partial CPU set.

Thank you for clarifying! Much appreciated _/\_

-- 
Thanks and Regards,
Prateek