Re: [PATCH RFC v2 08/16] vfio-user: get region info

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Sep 09, 2021 at 05:35:40AM +0000, John Johnson wrote:
> 
> 
> > On Sep 7, 2021, at 7:31 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > 
> > On Mon, Aug 16, 2021 at 09:42:41AM -0700, Elena Ufimtseva wrote:
> >> @@ -1514,6 +1515,16 @@ bool vfio_get_info_dma_avail(struct vfio_iommu_type1_info *info,
> >>     return true;
> >> }
> >> 
> >> +static int vfio_get_region_info_remfd(VFIODevice *vbasedev, int index)
> >> +{
> >> +    struct vfio_region_info *info;
> >> +
> >> +    if (vbasedev->regions == NULL || vbasedev->regions[index] == NULL) {
> >> +        vfio_get_region_info(vbasedev, index, &info);
> >> +    }
> > 
> > Maybe this will be called from other places in the future, but the
> > vfio_region_setup() caller below already invoked vfio_get_region_info()
> > so I'm not sure it's necessary to do this again?
> > 
> > Perhaps add an int *remfd argument to vfio_get_region_info(). That way
> > vfio_get_region_info_remfd() isn't necessary.
> > 
> 
> 	I think they could be combined, but the region capabilities are
> retrieved with separate calls to vfio_get_region_info_cap(), so I followed
> that precedent.
> 
> 
> >> @@ -2410,6 +2442,24 @@ int vfio_get_region_info(VFIODevice *vbasedev, int index,
> >>                          struct vfio_region_info **info)
> >> {
> >>     size_t argsz = sizeof(struct vfio_region_info);
> >> +    int fd = -1;
> >> +    int ret;
> >> +
> >> +    /* create region cache */
> >> +    if (vbasedev->regions == NULL) {
> >> +        vbasedev->regions = g_new0(struct vfio_region_info *,
> >> +                                   vbasedev->num_regions);
> >> +        if (vbasedev->proxy != NULL) {
> >> +            vbasedev->regfds = g_new0(int, vbasedev->num_regions);
> >> +        }
> >> +    }
> >> +    /* check cache */
> >> +    if (vbasedev->regions[index] != NULL) {
> >> +        *info = g_malloc0(vbasedev->regions[index]->argsz);
> >> +        memcpy(*info, vbasedev->regions[index],
> >> +               vbasedev->regions[index]->argsz);
> >> +        return 0;
> >> +    }
> > 
> > Why is it necessary to introduce a cache? Is it to avoid passing the
> > same fd multiple times?
> > 
> 
> 	Yes.  For polled regions, the server returns an FD with each
> message, so there would be an FD leak if I didn’t check that the region
> hadn’t already returned one.  Since I had to cache the FD anyway, I
> cached the whole struct.

If vfio_get_region_info() takes an int *fd argument then fd ownership
becomes explicit and the need for the cache falls away. Maybe Alex has a
preference for how to structure the code to track per-region fds.

Stefan

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