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Re: [Qemu-devel] [Xen-devel] [PATCH] qemu/xendisk: set maximum number of
From: |
Stefano Stabellini |
Subject: |
Re: [Qemu-devel] [Xen-devel] [PATCH] qemu/xendisk: set maximum number of grants to be used |
Date: |
Fri, 11 May 2012 18:07:42 +0100 |
User-agent: |
Alpine 2.00 (DEB 1167 2008-08-23) |
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