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Subject: Re: [PATCH 3/3] virtio-pci: Use cpumask_available to fix compilation
 error
From:   Christophe Marie Francois Dupont de Dinechin <cdupontd@redhat.com>
In-Reply-To: <20220415044657-mutt-send-email-mst@kernel.org>
Date:   Thu, 28 Apr 2022 11:48:01 +0200
Cc:     Christophe de Dinechin <dinechin@redhat.com>, trivial@kernel.org,
        Ben Segall <bsegall@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>, Mel Gorman <mgorman@suse.de>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org,
        Zhen Lei <thunder.leizhen@huawei.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
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Message-Id: <3D264F7F-624D-4E9D-A139-F1DB0CC6045C@redhat.com>
References: <20220414150855.2407137-1-dinechin@redhat.com>
 <20220414150855.2407137-4-dinechin@redhat.com>
 <20220415044657-mutt-send-email-mst@kernel.org>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Precedence: bulk


> On 15 Apr 2022, at 10:48, Michael S. Tsirkin <mst@redhat.com> wrote:
>=20
> On Thu, Apr 14, 2022 at 05:08:55PM +0200, Christophe de Dinechin =
wrote:
>> With GCC 12 and defconfig, we get the following error:
>>=20
>> |   CC      drivers/virtio/virtio_pci_common.o
>> | drivers/virtio/virtio_pci_common.c: In function =E2=80=98vp_del_vqs=E2=
=80=99:
>> | drivers/virtio/virtio_pci_common.c:257:29: error: the comparison =
will
>> |  always evaluate as =E2=80=98true=E2=80=99 for the pointer operand =
in
>> |  =E2=80=98vp_dev->msix_affinity_masks + (sizetype)((long unsigned =
int)i * 8)=E2=80=99
>> |  must not be NULL [-Werror=3Daddress]
>> |   257 |                         if (vp_dev->msix_affinity_masks[i])
>> |       |                             ^~~~~~
>>=20
>> This happens in the case where CONFIG_CPUMASK_OFFSTACK is not =
defined,
>> since we typedef cpumask_var_t as an array. The compiler is =
essentially
>> complaining that an array pointer cannot be NULL. This is not a very
>> important warning, but there is a function called cpumask_available =
that
>> seems to be defined just for that case, so the fix is easy.
>>=20
>> Signed-off-by: Christophe de Dinechin <christophe@dinechin.org>
>> Signed-off-by: Christophe de Dinechin <dinechin@redhat.com>
>=20
> There was an alternate patch proposed for this by
> Murilo Opsfelder Araujo. What do you think about that approach?

I responded on the other thread, but let me share the response here:

[to muriloo@linux.ibm.com]
Apologies for the delay in responding, broken laptop=E2=80=A6

In the case where CONFIG_CPUMASK_OFFSTACK is not defined, we have:

	typedef struct cpumask cpumask_var_t[1];

So that vp_dev->msix_affinity_masks[i] is statically not null (that=E2=80=99=
s the warning)
but also a static pointer, so not kfree-safe IMO.

>=20
>=20
>> ---
>> drivers/virtio/virtio_pci_common.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>=20
>> diff --git a/drivers/virtio/virtio_pci_common.c =
b/drivers/virtio/virtio_pci_common.c
>> index d724f676608b..5c44a2f13c93 100644
>> --- a/drivers/virtio/virtio_pci_common.c
>> +++ b/drivers/virtio/virtio_pci_common.c
>> @@ -254,7 +254,7 @@ void vp_del_vqs(struct virtio_device *vdev)
>>=20
>> 	if (vp_dev->msix_affinity_masks) {
>> 		for (i =3D 0; i < vp_dev->msix_vectors; i++)
>> -			if (vp_dev->msix_affinity_masks[i])
>> +			if =
(cpumask_available(vp_dev->msix_affinity_masks[i]))
>> 				=
free_cpumask_var(vp_dev->msix_affinity_masks[i]);
>> 	}
>>=20
>> --=20
>> 2.35.1
>=20