Re: [PATCH v10 4/6] x86/sev: Add support to perform RMP optimizations asynchronously

[K Prateek Nayak]

Hello Ashish,

On 6/30/2026 11:41 PM, Ashish Kalra wrote:
> +	/*
> +	 * RMPOPT scans the RMP table, stores the result of the scan in the
> +	 * reserved processor memory. The RMP scan is the most expensive
> +	 * part. If a second RMPOPT occurs, it can skip the expensive scan
> +	 * if they can see a cached result in the reserved processor memory.
> +	 *
> +	 * Do RMPOPT on one CPU alone. Then, follow that up with RMPOPT
> +	 * on every other primary thread. Followers are "designed to"
> +	 * skip the scan if they see the "cached" scan results.
> +	 *
> +	 * Pin the worker to the current CPU for the leader loop so that
> +	 * this_cpu remains valid and the RMPOPT instruction executes on
> +	 * the correct CPU.  Use migrate_disable() rather than get_cpu() to
> +	 * prevent migration while still allowing preemption.
> +	 */
> +	migrate_disable();
> +	this_cpu = smp_processor_id();
> +
> +	cpumask_andnot(follower_mask, rmpopt_cpumask,
> +		       topology_sibling_cpumask(this_cpu));
> +
> +	for (pa = rmpopt_pa_start; pa < rmpopt_pa_end; pa += SZ_1G) {
> +		rmpopt(pa);
> +		cond_resched();
> +	}
> +	migrate_enable();
> +
> +	/*
> +	 * Followers: run RMPOPT on remaining cores.  CPUs cannot go offline
> +	 * while SNP is active, so the follower set stays valid across the
> +	 * scan and cpus_read_lock() is uncontended.
> +	 */
> +	scoped_guard(cpus_read_lock) {

We can only reach here after disabling hotplug. Do we still to hold the
cpus_read_lock?

> +		for (pa = rmpopt_pa_start; pa < rmpopt_pa_end; pa += SZ_1G) {
> +			on_each_cpu_mask(follower_mask, rmpopt_smp,
> +					 (void *)pa, true);
> +
> +			/* Give a chance for other threads to run */
> +			cond_resched();

Ever since commit 7dadeaa6e851 ("sched: Further restrict the preemption
modes"), one cannot select PREEMPT_VOLUNTARY on x86 and cond_resched()
is a nop with all the other preemption models (PREEMPT_NONE will not
preempt a kernel thread; PREEMPT_LAZY / PREEMPT / PREEMPT_RT all make
the kernel fully preemptible except for sections with
preempt_count != 0)

You can drop these cond_resched() calls in your respin.

> +		}
> +	}
> +
> +	free_cpumask_var(follower_mask);
> +}
> +

-- 
Thanks and Regards,
Prateek