On 8/31/26 13:58, Ilya Leoshkevich wrote:
> libvirt assigns affinities to QEMU threads only on domain start,
> therefore hot plugged host CPUs cannot be used. Restore the logic from
> commit 283e29043423 ("qemu: Allow use of hot plugged host CPUs if no
> affinity set"): when the affinity that would be set covers all the
> online CPUs anyway, do not set it at all.
>
> qemuProcessGetAllCpuAffinity() already knows when this is the case,
> so let it return NULL, which the callers already treat as "leave the
> affinity alone".
>
> Fixes: f136b83139c6 ("qemu: Rework setting process affinity")
> Signed-off-by: Ilya Leoshkevich <[email protected]>
> ---
> src/qemu/qemu_process.c | 24 ++++++++++++++++++++----
> 1 file changed, 20 insertions(+), 4 deletions(-)
>
> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
> index b2506edce0..413ec0622d 100644
> --- a/src/qemu/qemu_process.c
> +++ b/src/qemu/qemu_process.c
> @@ -2567,30 +2567,46 @@ qemuProcessDetectIOThreadPIDs(virDomainObj *vm,
> }
>
>
> +/**
> + * qemuProcessGetAllCpuAffinity:
> + * @cpumapRet: returned CPU affinity map
> + *
> + * Sets @cpumapRet to the online CPUs minus the isolated ones.
> + *
> + * In case there is nothing to exclude (no isolated CPUs, or no overlap with
> + * online CPUs), sets @cpumapRet to NULL instead, indicating to the caller
> that
> + * it should not call sched_setaffinity(), which would prevent the usage of
> + * CPUs that are hot plugged later on.
> + *
> + * Returns: 0 on success, -1 on error.
> + */
> static int
> qemuProcessGetAllCpuAffinity(virBitmap **cpumapRet)
> {
> g_autoptr(virBitmap) isolCpus = NULL;
> + g_autoptr(virBitmap) cpumap = NULL;
>
> *cpumapRet = NULL;
>
> if (!virHostCPUHasBitmap())
> return 0;
>
> - if (!(*cpumapRet = virHostCPUGetOnlineBitmap()))
> + if (!(cpumap = virHostCPUGetOnlineBitmap()))
> return -1;
>
> if (virHostCPUGetIsolated(&isolCpus) < 0)
> return -1;
>
> - if (isolCpus) {
> + if (isolCpus && virBitmapOverlaps(cpumap, isolCpus)) {
> g_autofree char *isolCpusStr = virBitmapFormat(isolCpus);
> - g_autofree char *cpumapRetStr = virBitmapFormat(*cpumapRet);
> + g_autofree char *cpumapRetStr = virBitmapFormat(cpumap);
>
> VIR_INFO("Subtracting isolated CPUs %1$s from online CPUs %2$s",
> isolCpusStr, cpumapRetStr);
>
> - virBitmapSubtract(*cpumapRet, isolCpus);
> + virBitmapSubtract(cpumap, isolCpus);
> +
> + *cpumapRet = g_steal_pointer(&cpumap);
> }
>
> return 0;
Nice catch!
Michal
