Re: [Qemu-devel] [Xen-devel] [PATCH] qemu/xendisk: set maximum number of grants to be used

[Stefano Stabellini]

On Fri, 11 May 2012, Jan Beulich wrote:
> >>> On 11.05.12 at 09:19, "Jan Beulich" <address@hidden> wrote:
> > Legacy (non-pvops) gntdev drivers may require this to be done when the
> > number of grants intended to be used simultaneously exceeds a certain
> > driver specific default limit.
> > 
> > Signed-off-by: Jan Beulich <address@hidden>
> > 
> > --- a/hw/xen_disk.c
> > +++ b/hw/xen_disk.c
> > @@ -536,6 +536,10 @@ static void blk_alloc(struct XenDevice *
> >      if (xen_mode != XEN_EMULATE) {
> >          batch_maps = 1;
> >      }
> > +    if (xc_gnttab_set_max_grants(xendev->gnttabdev,
> > +            max_requests * BLKIF_MAX_SEGMENTS_PER_REQUEST + 1) < 0)
> 
> In more extensive testing it appears that very rarely this value is still
> too low:
> 
> xen be: qdisk-768: can't map 11 grant refs (Cannot allocate memory, 342 maps)
> 
> 342 + 11 = 353 > 352 = 32 * 11
> 
> Could someone help out here? I first thought this might be due to
> use_aio being non-zero, but ioreq_start() doesn't permit more than
> max_requests struct ioreqs-s to be around.

Actually 342 + 11 = 353, that should be still OK because it is equal to
32 * 11 + 1, where the additional 1 is for the ring, right?


> Additionally, shouldn't the driver be smarter and gracefully handle
> grant mapping failures (as the per-domain map track table in the
> hypervisor is a finite resource)?

yes, probably