Re: [PATCH v9 3/6] x86/sev: Disable CPU hotplug while SNP is active

[K Prateek Nayak]

Hello Boris,

On 6/27/2026 10:11 AM, Borislav Petkov wrote:
> You could also block hotplug for the time being by grabbing cpus_read_lock().
> And only when you know you are all clear to disable hotplug, then you can do
> that in the end and drop the hotplug lock.

This is bad idea because it'll stall the tasks trying to do a hotplug
until the last SNP VM exits. Instead of simply getting an -EBUSY, the
users will start seeing a hung task splats in dmesg.

Also, since the last VM has to re-enable hotplug, you'll need a
up_read_non_owner() variant for cpus_read_unlock() to unlock the rwsem
from a different thread compared to the one that locks.

I think cpu_hotplug_disable() is the correct way to go forward but if
you are not a fan of the global "snp_cpu_hotplug_disabled" flag, maybe
it can be turned into an indicator like "snp_initialized" in
"struct sev_device". Thoughts?

-- 
Thanks and Regards,
Prateek