Re: [PATCH 1/2] virhostcpu: Introduce virHostCPUGetIsolated()

[Martin Kletzander <mkletzan@xxxxxxxxxx>]

On Mon, Apr 22, 2024 at 01:38:38PM +0200, Michal Privoznik wrote:
> This is a helper that parses /sys/devices/system/cpu/isolated
> into a virBitmap. It's going to be needed soon.
>
> Signed-off-by: Michal Privoznik <mprivozn@xxxxxxxxxx>
> ---
> src/libvirt_private.syms |  1 +
> src/util/virhostcpu.c    | 21 +++++++++++++++++++++
> src/util/virhostcpu.h    |  1 +
> 3 files changed, 23 insertions(+)
>
> diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
> index 839fe4f545..ed85f022e4 100644
> --- a/src/libvirt_private.syms
> +++ b/src/libvirt_private.syms
> @@ -2502,6 +2502,7 @@ virHostCPUGetCount;
> virHostCPUGetCPUID;
> virHostCPUGetHaltPollTime;
> virHostCPUGetInfo;
> +virHostCPUGetIsolated;
> virHostCPUGetKVMMaxVCPUs;
> virHostCPUGetMap;
> virHostCPUGetMicrocodeVersion;
> diff --git a/src/util/virhostcpu.c b/src/util/virhostcpu.c
> index 01de69c0d1..e61eb12414 100644
> --- a/src/util/virhostcpu.c
> +++ b/src/util/virhostcpu.c
> @@ -1151,6 +1151,27 @@ virHostCPUGetAvailableCPUsBitmap(void)
>     return g_steal_pointer(&bitmap);
> }
>
> +/**
> + * virHostCPUGetIsolated:
> + *
> + * Returns a bitmap of isolated CPUs (e.g. those passed to isolcpus= kernel
> + * cmdline).
> + */
> +virBitmap *
> +virHostCPUGetIsolated(void)
> +{

So this function will fail on anything non-Linux and we probably do not
want that.  If the platform does not support isolcpus, then it is fine
to return an empty bitmap.

> +#ifdef __linux__
> +    virBitmap *ret = NULL;
> +
> +    virFileReadValueBitmap(&ret, "%s/cpu/isolated", SYSFS_SYSTEM_PATH);
> +

And this will fail (return NULL) on any system without isolcpus=, which
is also probably something we do not want.

You can also use the return value of virFileReadValueBitmap() to see if
the file exists, but I'd even go as far as just using the bitmap if it
returned non-NULL, otherwise carrying on.

> +    return ret;
> +#else
> +    virReportError(VIR_ERR_NO_SUPPORT, "%s",
> +                   _("map of isolated CPUs not implemented on this platform"));
> +    return NULL;
> +#endif
> +}
>
> #if WITH_LINUX_KVM_H && defined(KVM_CAP_PPC_SMT)
>
> diff --git a/src/util/virhostcpu.h b/src/util/virhostcpu.h
> index d7e09bff22..e7b15a2649 100644
> --- a/src/util/virhostcpu.h
> +++ b/src/util/virhostcpu.h
> @@ -43,6 +43,7 @@ bool virHostCPUHasBitmap(void);
> virBitmap *virHostCPUGetPresentBitmap(void);
> virBitmap *virHostCPUGetOnlineBitmap(void);
> virBitmap *virHostCPUGetAvailableCPUsBitmap(void);
> +virBitmap *virHostCPUGetIsolated(void);
>
> int virHostCPUGetCount(void);
> int virHostCPUGetThreadsPerSubcore(virArch arch) G_NO_INLINE;
> --
> 2.43.2
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