Re: [PATCH v8 4/7] x86/sev: Add support to perform RMP optimizations asynchronously

[K Prateek Nayak]

Hello Ashish,

On 6/16/2026 1:19 AM, 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.
> +	 */
> +	cpumask_copy(follower_mask, &rmpopt_cpumask);

rmpopt_cpumask is constructed after hotplug is disabled but ...

> +
> +	/*
> +	 * 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();
> +
> +	if (cpumask_test_cpu(this_cpu, follower_mask)) {
> +		/*
> +		 * Current CPU is a primary thread in rmpopt_cpumask.
> +		 * Run leader locally and remove from follower mask.
> +		 */
> +		cpumask_clear_cpu(this_cpu, follower_mask);
> +
> +		for (pa = rmpopt_pa_start; pa < rmpopt_pa_end; pa += SZ_1G) {
> +			rmpopt(pa);
> +			cond_resched();
> +		}
> +	} else if (cpumask_intersects(topology_sibling_cpumask(this_cpu),
> +				      follower_mask)) {
> +		/*
> +		 * Current CPU is a sibling thread whose primary is in
> +		 * rmpopt_cpumask.  RMPOPT_BASE MSR is per-core, so it
> +		 * is safe to run the leader locally.  Remove the sibling's
> +		 * primary from the follower mask as this core is already
> +		 * covered by the leader.
> +		 */
> +		cpumask_andnot(follower_mask, follower_mask,
> +			       topology_sibling_cpumask(this_cpu));
> +
> +		for (pa = rmpopt_pa_start; pa < rmpopt_pa_end; pa += SZ_1G) {
> +			rmpopt(pa);
> +			cond_resched();
> +		}
> +	} else {
> +		/*
> +		 * Current CPU does not have RMPOPT_BASE MSR programmed.
> +		 * Pick an explicit leader from the cpumask to avoid #UD.
> +		 * Use work_on_cpu() to run in process context on the leader,
> +		 * avoiding IPI latency.
> +		 */

... this_cpu is neither in the "rmpopt_cpumask", nor is any of its
siblings on "rmpopt_cpumask".

How does that happen?

> +		int leader_cpu = cpumask_first(follower_mask);
> +
> +		if (WARN_ON_ONCE(leader_cpu >= nr_cpu_ids)) {
> +			migrate_enable();
> +			goto out;
> +		}
> +
> +		cpumask_clear_cpu(leader_cpu, follower_mask);
> +
> +		/* Release migration pin before work_on_cpu(). */
> +		migrate_enable();
> +
> +		work_on_cpu(leader_cpu, rmpopt_leader_fn, NULL);

This creates a delayed work and also waits for it to finish execution
which will add more latency than a simple IPI if the comment about IPI
latency above is accurate.

I think there is some corner case in construction of the
"rmpopt_cpumask" that requires this not-so-pretty else block. Can you
elaborate why this is required?

Perhaps the "rmpopt_cpumask" construction needs:

    for_each_online_cpu(cpu) {
        /* Nominate the first CPU on the sibling mask for RMPOPT */
        if (cpu != cpumask_first(topology_sibling_cpumask(cpu)))
            continue;
        cpumask_set_cpu(cpu, &rmpopt_cpumask);
    }


and all you need here is:

    /* Do RMPOPt for local core */
    for (pa = rmpopt_pa_start; pa < rmpopt_pa_end; pa += SZ_1G)
        rmpopt(pa);

    /* Skip this core from concurrent RMPOPT */
    cpumask_and_not(follower_mask, &rmpopt_cpumask, topology_sibling_cpumask(cpu));

No?

> +
> +		goto followers;
> +	}
> +
> +	migrate_enable();
> +
-- 
Thanks and Regards,
Prateek