[libvirt] [PATCH] qemu: Start domain on a node without cpu affinity

[Peter Krempa]

On Wed, Aug 22, 2018 at 00:31:29 +0300, Roman Bolshakov wrote:
> libvirt affinity wrappers don't support macOS Thread Affinity API:
> https://developer.apple.com/library/archive/releasenotes/Performance/RN-AffinityAPI/index.html
> 
> virProcessSetAffinity stub prevents libvirt from starting a qemu domain
> on macOS:
> 
> $ virsh start vm
> error: Failed to start domain vm
> error: Process CPU affinity is not supported on this platform: Function not implemented
> 
> With the patch a VM can be started on macOS but some affinity-related
> commands will return errors:
> 
> $ virsh vcpuinfo vm
> error: Requested operation is not valid: cpu affinity is not supported
> 
> $ virsh vcpupin vm
> VCPU: CPU Affinity
> ----------------------------------
>    0: 0-7
> 
> $ virsh vcpupin vm --live --vcpu 0 --cpulist 7
> error: operation failed: Virt type 'qemu' does not support vCPU pinning
> 
> $ virsh emulatorpin vm
> emulator: CPU Affinity
> ----------------------------------
>        *: 0-7
> 
> $ virsh emulatorpin vm --live --cpulist 7
> error: Requested operation is not valid: cpu affinity is not supported
> 
> The patch also fixes virmacmaptest on macOS
> 
> Signed-off-by: Roman Bolshakov <r.bolshakov at yadro.com>
> ---
>  src/qemu/qemu_driver.c | 6 ++++++
>  src/util/virprocess.c  | 2 +-
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
> index 21e9e87ddd..2e225b1ede 100644
> --- a/src/qemu/qemu_driver.c
> +++ b/src/qemu/qemu_driver.c
> @@ -5245,6 +5245,12 @@ qemuDomainPinEmulator(virDomainPtr dom,
>      if (virDomainPinEmulatorEnsureACL(dom->conn, vm->def, flags) < 0)
>          goto cleanup;
>  
> +    if (!qemuDomainHasVcpuPids(vm)) {
> +        virReportError(VIR_ERR_OPERATION_INVALID,
> +                       "%s", _("cpu affinity is not supported"));
> +        goto cleanup;
> +    }
> +
>      if (qemuDomainObjBeginJob(driver, vm, QEMU_JOB_MODIFY) < 0)
>          goto cleanup;
>  
> diff --git a/src/util/virprocess.c b/src/util/virprocess.c
> index 3988f5546c..7eaafd29f2 100644
> --- a/src/util/virprocess.c
> +++ b/src/util/virprocess.c
> @@ -598,7 +598,7 @@ int virProcessSetAffinity(pid_t pid ATTRIBUTE_UNUSED,
>  {
>      virReportSystemError(ENOSYS, "%s",
>                           _("Process CPU affinity is not supported on this platform"));
> -    return -1;
> +    return 0;

This is not a good idea. The caller should make sure that it does not
call this function if we need to support such a code path.

If you blindly return 0 here other callers might get confused.

Also setting an error and returning success is generally wrong.
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