Re: [PATCH 3/3] virtio-pci: Use cpumask_available to fix compilation error

[Michael S. Tsirkin]

On Thu, Apr 28, 2022 at 11:48:01AM +0200, Christophe Marie Francois Dupont de Dinechin wrote:
> 
> 
> > On 15 Apr 2022, at 10:48, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> > 
> > On Thu, Apr 14, 2022 at 05:08:55PM +0200, Christophe de Dinechin wrote:
> >> With GCC 12 and defconfig, we get the following error:
> >> 
> >> |   CC      drivers/virtio/virtio_pci_common.o
> >> | drivers/virtio/virtio_pci_common.c: In function ‘vp_del_vqs’:
> >> | drivers/virtio/virtio_pci_common.c:257:29: error: the comparison will
> >> |  always evaluate as ‘true’ for the pointer operand in
> >> |  ‘vp_dev->msix_affinity_masks + (sizetype)((long unsigned int)i * 8)’
> >> |  must not be NULL [-Werror=address]
> >> |   257 |                         if (vp_dev->msix_affinity_masks[i])
> >> |       |                             ^~~~~~
> >> 
> >> 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.
> >> 
> >> Signed-off-by: Christophe de Dinechin <christophe@xxxxxxxxxxxx>
> >> Signed-off-by: Christophe de Dinechin <dinechin@xxxxxxxxxx>
> > 
> > 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@xxxxxxxxxxxxx]
> Apologies for the delay in responding, broken laptop…
> 
> 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’s the warning)
> but also a static pointer, so not kfree-safe IMO.


Not sure I understand what you are saying here.

> > 
> > 
> >> ---
> >> drivers/virtio/virtio_pci_common.c | 2 +-
> >> 1 file changed, 1 insertion(+), 1 deletion(-)
> >> 
> >> 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)
> >> 
> >> 	if (vp_dev->msix_affinity_masks) {
> >> 		for (i = 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]);
> >> 	}
> >> 
> >> -- 
> >> 2.35.1
> >