Re: [PATCH 4/7] KVM: selftests: Compute node masks on-demand in xAPIC IPI test

[Garg, Shivank]

On Wed, 2026-09-02 at 17:16 -0700, Sean Christopherson wrote:
> Compute the node masks for the source (from) and destination (to) NUMA
> nodes in the xAPIC IPI test instead of pre-filling an array of masks with
> node per mask.  Computing the mask on-demand is technically slower, but
> doesn't require a large-ish on-stack array, and more importantly allows the
> test to use a generic "get next NUMA node" API without having to commit all
> of KVM selftests to using a large array of single-bit nodemasks.
> 
> Implement said API as a common KVM NUMA API so that it can be used by other
> tests, e.g. in guest_memfd tests.  Deliberately make @from "exclusive" as
> the anticipated usage in KVM selftests is to select the next, *different*
> node, i.e. so that users don't have to copy+paste code to assert that the
> found node is different than the starting node.  The obvious downside is
> that implementing the exclusive logic forces callers to pass -1 instead of
> 0 when the goal is to find the first node in the mask, but that arguably
> yields more intuitive code anyways.
> 
> From an overall test functionality/coverage perspective, no functional
> change intended (the walking pattern of node migration should be unchanged).
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  tools/testing/selftests/kvm/include/numaif.h  | 21 +++++++++++-
>  .../selftests/kvm/x86/xapic_ipi_test.c        | 32 ++++++-------------
>  2 files changed, 30 insertions(+), 23 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/include/numaif.h b/tools/testing/selftests/kvm/include/numaif.h
> index 71f261eafc90..299dddff2729 100644
> --- a/tools/testing/selftests/kvm/include/numaif.h
> +++ b/tools/testing/selftests/kvm/include/numaif.h
> @@ -6,7 +6,7 @@
>  
>  #include <dirent.h>
>  
> -#include <linux/bitops.h>
> +#include <linux/bitmap.h>
>  #include <linux/mempolicy.h>
>  
>  #include "kvm_syscalls.h"
> @@ -41,6 +41,25 @@ KVM_SYSCALL_DEFINE(mbind, 6, void *, addr, unsigned long, size, int, mode,
>   */
>  #define MAXNODE_FOR_MASK(mask) (BITS_PER_TYPE(mask) + 1)
>  
> +/*
> + * Return the node ID of the next NUMA node in the mask, starting at @from+1.
> + * Guarantees a node is found, and that the found node is not @from.  Pass -1
> + * to find the first node in the mask.
> + */
> +static inline int kvm_get_next_numa_node(unsigned long nodemask, int from)
> +{
> +	const unsigned long nr_bits = BITS_PER_TYPE(nodemask);
> +	int to;
> +
> +	to = find_next_bit(&nodemask, nr_bits, from + 1);
> +	if (to == nr_bits)
> +		to = find_next_bit(&nodemask, nr_bits, 0);
> +
> +	TEST_ASSERT(to != nr_bits && to != from,
> +		    "Unabled to find second NUMA node (from = %d, to = %d)", from, to);

typo: Unable


> +	return to;
> +}
> +
>  static inline int get_max_numa_node(void)
>  {
>  	struct dirent *de;
> diff --git a/tools/testing/selftests/kvm/x86/xapic_ipi_test.c b/tools/testing/selftests/kvm/x86/xapic_ipi_test.c
> index 9d1dfad4efa6..7144ad833ae0 100644
> --- a/tools/testing/selftests/kvm/x86/xapic_ipi_test.c
> +++ b/tools/testing/selftests/kvm/x86/xapic_ipi_test.c
> @@ -233,13 +233,10 @@ void do_migrations(struct test_data_page *data, int run_secs, int delay_usecs,
>  {
>  	long pages_not_moved;
>  	unsigned long nodemask = 0;
> -	unsigned long nodemasks[BITS_PER_TYPE(nodemask)];
>  	int nodes = 0;
>  	time_t start_time, last_update, now;
>  	time_t interval_secs = 1;
> -	int i;
>  	int from, to;
> -	unsigned long bit;
>  	u64 hlt_count;
>  	u64 wake_count;
>  	u64 ipis_sent;
> @@ -255,24 +252,15 @@ void do_migrations(struct test_data_page *data, int run_secs, int delay_usecs,
>  		"(each 1-bit indicates node is present): %#lx\n",
>  		BITS_PER_TYPE(nodemask), nodemask);
>  
> -	/* Init array of masks containing a single-bit in each, one for each
> -	 * available node. migrate_pages called below requires specifying nodes
> -	 * as bit masks.
> -	 */
> -	for (i = 0, bit = 1; i < BITS_PER_TYPE(nodemask); i++, bit <<= 1) {
> -		if (nodemask & bit) {
> -			nodemasks[nodes] = nodemask & bit;
> -			nodes++;
> -		}
> -	}
> -
> +	nodes = __builtin_popcountl(nodemask);
>  	TEST_ASSERT(nodes > 1,
>  		    "Did not find at least 2 numa nodes. Can't do migration");
>  
>  	fprintf(stderr, "Migrating amongst %d nodes found\n", nodes);
>  
> -	from = 0;
> -	to = 1;
> +	from = kvm_get_next_numa_node(nodemask, -1);
> +	to = kvm_get_next_numa_node(nodemask, from);
> +
>  	start_time = time(NULL);
>  	last_update = start_time;
>  
> @@ -281,6 +269,9 @@ void do_migrations(struct test_data_page *data, int run_secs, int delay_usecs,
>  	wake_count = data->wake_count;
>  
>  	while ((int)(time(NULL) - start_time) < run_secs) {
> +		unsigned long from_mask = BIT(from);
> +		unsigned long to_mask = BIT(to);
> +
>  		data->migrations_attempted++;
>  
>  		/*
> @@ -291,9 +282,8 @@ void do_migrations(struct test_data_page *data, int run_secs, int delay_usecs,
>  		 * KVM_CREATE_VCPU ioctl. If that assumption ever changes this
>  		 * test may break or give a false positive signal.
>  		 */
> -		pages_not_moved = migrate_pages(0, MAXNODE_FOR_MASK(nodemasks[from]),
> -						&nodemasks[from],
> -						&nodemasks[to]);
> +		pages_not_moved = migrate_pages(0, MAXNODE_FOR_MASK(from_mask),
> +						&from_mask, &to_mask);
>  		if (pages_not_moved < 0)
>  			fprintf(stderr,
>  				"migrate_pages failed, errno=%d\n", errno);
> @@ -305,9 +295,7 @@ void do_migrations(struct test_data_page *data, int run_secs, int delay_usecs,
>  			data->migrations_completed++;
>  
>  		from = to;
> -		to++;
> -		if (to == nodes)
> -			to = 0;
> +		to = kvm_get_next_numa_node(nodemask, from);
>  
>  		now = time(NULL);
>  		if (((now - start_time) % interval_secs == 0) &&

LGTM

Reviewed-by: Shivank Garg <shivankg@xxxxxxx>

Thanks,
Shivank