Re: [PULL 3/5] hw/core: allow parameter=1 for SMP topology on any machine

["Daniel P. Berrangé" <berrange@redhat.com>]

Hi Michael,

This patch fixes a regression that was introduced in QEMU 9.0,
reported by yet another user at:

  https://gitlab.com/qemu-project/qemu/-/issues/2420

Could you pull this patch into stable-9.0.  If you think testing
is important for stable, the following patch adds further unit
testing coverage too.

Daniel

On Fri, May 17, 2024 at 05:02:25PM +0200, Philippe Mathieu-Daudé wrote:
> From: Daniel P. Berrangé <berrange@redhat.com>
> 
> This effectively reverts
> 
>   commit 54c4ea8f3ae614054079395842128a856a73dbf9
>   Author: Zhao Liu <zhao1.liu@intel.com>
>   Date:   Sat Mar 9 00:01:37 2024 +0800
> 
>     hw/core/machine-smp: Deprecate unsupported "parameter=1" SMP configurations
> 
> but is not done as a 'git revert' since the part of the changes to the
> file hw/core/machine-smp.c which add 'has_XXX' checks remain desirable.
> Furthermore, we have to tweak the subsequently added unit test to
> account for differing warning message.
> 
> The rationale for the original deprecation was:
> 
>   "Currently, it was allowed for users to specify the unsupported
>    topology parameter as "1". For example, x86 PC machine doesn't
>    support drawer/book/cluster topology levels, but user could specify
>    "-smp drawers=1,books=1,clusters=1".
> 
>    This is meaningless and confusing, so that the support for this kind
>    of configurations is marked deprecated since 9.0."
> 
> There are varying POVs on the topic of 'unsupported' topology levels.
> 
> It is common to say that on a system without hyperthreading, that there
> is always 1 thread. Likewise when new CPUs introduced a concept of
> multiple "dies', it was reasonable to say that all historical CPUs
> before that implicitly had 1 'die'. Likewise for the more recently
> introduced 'modules' and 'clusters' parameter'. From this POV, it is
> valid to set 'parameter=1' on the -smp command line for any machine,
> only a value > 1 is strictly an error condition.
> 
> It doesn't cause any functional difficulty for QEMU, because internally
> the QEMU code is itself assuming that all "unsupported" parameters
> implicitly have a value of '1'.
> 
> At the libvirt level, we've allowed applications to set 'parameter=1'
> when configuring a guest, and pass that through to QEMU.
> 
> Deprecating this creates extra difficulty for because there's no info
> exposed from QEMU about which machine types "support" which parameters.
> Thus, libvirt can't know whether it is valid to pass 'parameter=1' for
> a given machine type, or whether it will trigger deprecation messages.
> 
> Since there's no apparent functional benefit to deleting this deprecated
> behaviour from QEMU, and it creates problems for consumers of QEMU,
> remove this deprecation.
> 
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
> Reviewed-by: Ján Tomko <jtomko@redhat.com>
> Message-ID: <20240513123358.612355-2-berrange@redhat.com>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> ---
>  hw/core/machine-smp.c       | 84 ++++++++++++-------------------------
>  tests/unit/test-smp-parse.c |  8 ++--
>  2 files changed, 31 insertions(+), 61 deletions(-)
> 
> diff --git a/hw/core/machine-smp.c b/hw/core/machine-smp.c
> index 2b93fa99c9..5d8d7edcbd 100644
> --- a/hw/core/machine-smp.c
> +++ b/hw/core/machine-smp.c
> @@ -118,76 +118,46 @@ void machine_parse_smp_config(MachineState *ms,
>      }
>  
>      /*
> -     * If not supported by the machine, a topology parameter must be
> -     * omitted.
> +     * If not supported by the machine, a topology parameter must
> +     * not be set to a value greater than 1.
>       */
> -    if (!mc->smp_props.modules_supported && config->has_modules) {
> -        if (config->modules > 1) {
> -            error_setg(errp, "modules not supported by this "
> -                       "machine's CPU topology");
> -            return;
> -        } else {
> -            /* Here modules only equals 1 since we've checked zero case. */
> -            warn_report("Deprecated CPU topology (considered invalid): "
> -                        "Unsupported modules parameter mustn't be "
> -                        "specified as 1");
> -        }
> +    if (!mc->smp_props.modules_supported &&
> +        config->has_modules && config->modules > 1) {
> +        error_setg(errp,
> +                   "modules > 1 not supported by this machine's CPU topology");
> +        return;
>      }
>      modules = modules > 0 ? modules : 1;
>  
> -    if (!mc->smp_props.clusters_supported && config->has_clusters) {
> -        if (config->clusters > 1) {
> -            error_setg(errp, "clusters not supported by this "
> -                       "machine's CPU topology");
> -            return;
> -        } else {
> -            /* Here clusters only equals 1 since we've checked zero case. */
> -            warn_report("Deprecated CPU topology (considered invalid): "
> -                        "Unsupported clusters parameter mustn't be "
> -                        "specified as 1");
> -        }
> +    if (!mc->smp_props.clusters_supported &&
> +        config->has_clusters && config->clusters > 1) {
> +        error_setg(errp,
> +                   "clusters > 1 not supported by this machine's CPU topology");
> +        return;
>      }
>      clusters = clusters > 0 ? clusters : 1;
>  
> -    if (!mc->smp_props.dies_supported && config->has_dies) {
> -        if (config->dies > 1) {
> -            error_setg(errp, "dies not supported by this "
> -                       "machine's CPU topology");
> -            return;
> -        } else {
> -            /* Here dies only equals 1 since we've checked zero case. */
> -            warn_report("Deprecated CPU topology (considered invalid): "
> -                        "Unsupported dies parameter mustn't be "
> -                        "specified as 1");
> -        }
> +    if (!mc->smp_props.dies_supported &&
> +        config->has_dies && config->dies > 1) {
> +        error_setg(errp,
> +                   "dies > 1 not supported by this machine's CPU topology");
> +        return;
>      }
>      dies = dies > 0 ? dies : 1;
>  
> -    if (!mc->smp_props.books_supported && config->has_books) {
> -        if (config->books > 1) {
> -            error_setg(errp, "books not supported by this "
> -                       "machine's CPU topology");
> -            return;
> -        } else {
> -            /* Here books only equals 1 since we've checked zero case. */
> -            warn_report("Deprecated CPU topology (considered invalid): "
> -                        "Unsupported books parameter mustn't be "
> -                        "specified as 1");
> -        }
> +    if (!mc->smp_props.books_supported &&
> +        config->has_books && config->books > 1) {
> +        error_setg(errp,
> +                   "books > 1 not supported by this machine's CPU topology");
> +        return;
>      }
>      books = books > 0 ? books : 1;
>  
> -    if (!mc->smp_props.drawers_supported && config->has_drawers) {
> -        if (config->drawers > 1) {
> -            error_setg(errp, "drawers not supported by this "
> -                       "machine's CPU topology");
> -            return;
> -        } else {
> -            /* Here drawers only equals 1 since we've checked zero case. */
> -            warn_report("Deprecated CPU topology (considered invalid): "
> -                        "Unsupported drawers parameter mustn't be "
> -                        "specified as 1");
> -        }
> +    if (!mc->smp_props.drawers_supported &&
> +        config->has_drawers && config->drawers > 1) {
> +        error_setg(errp,
> +                   "drawers > 1 not supported by this machine's CPU topology");
> +        return;
>      }
>      drawers = drawers > 0 ? drawers : 1;
>  
> diff --git a/tests/unit/test-smp-parse.c b/tests/unit/test-smp-parse.c
> index 8994337e12..56165e6644 100644
> --- a/tests/unit/test-smp-parse.c
> +++ b/tests/unit/test-smp-parse.c
> @@ -337,21 +337,21 @@ static const struct SMPTestData data_generic_invalid[] = {
>      {
>          /* config: -smp 2,dies=2 */
>          .config = SMP_CONFIG_WITH_DIES(T, 2, F, 0, T, 2, F, 0, F, 0, F, 0),
> -        .expect_error = "dies not supported by this machine's CPU topology",
> +        .expect_error = "dies > 1 not supported by this machine's CPU topology",
>      }, {
>          /* config: -smp 2,clusters=2 */
>          .config = SMP_CONFIG_WITH_CLUSTERS(T, 2, F, 0, T, 2, F, 0, F, 0, F, 0),
> -        .expect_error = "clusters not supported by this machine's CPU topology",
> +        .expect_error = "clusters > 1 not supported by this machine's CPU topology",
>      }, {
>          /* config: -smp 2,books=2 */
>          .config = SMP_CONFIG_WITH_BOOKS_DRAWERS(T, 2, F, 0, T, 2, F,
>                                                  0, F, 0, F, 0, F, 0),
> -        .expect_error = "books not supported by this machine's CPU topology",
> +        .expect_error = "books > 1 not supported by this machine's CPU topology",
>      }, {
>          /* config: -smp 2,drawers=2 */
>          .config = SMP_CONFIG_WITH_BOOKS_DRAWERS(T, 2, T, 2, F, 0, F,
>                                                  0, F, 0, F, 0, F, 0),
> -        .expect_error = "drawers not supported by this machine's CPU topology",
> +        .expect_error = "drawers > 1 not supported by this machine's CPU topology",
>      }, {
>          /* config: -smp 8,sockets=2,cores=4,threads=2,maxcpus=8 */
>          .config = SMP_CONFIG_GENERIC(T, 8, T, 2, T, 4, T, 2, T, 8),
> -- 
> 2.41.0
> 

With regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|