Re: [Xen-devel] [PATCH, v2] qemu/xendisk: set maximum number of grants to be used

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 31 May 2012, Jan Beulich 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.
> 
> Change in v2: Double the number requested, as we need to account for
> the allocations needing to happen in contiguous chunks. The worst case
> number would be max_req * max_seg + (max_req - 1) * (max_seg - 1) + 1,
> but in order to keep things simple just use 2 * max_req * max_seg.

Could you please add a brief explanation like this one as a comment in
the code below?


> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> --- a/hw/xen_disk.c
> +++ b/hw/xen_disk.c
> @@ -548,6 +548,11 @@ static void blk_alloc(struct XenDevice *
>      if (xen_mode != XEN_EMULATE) {
>          batch_maps = 1;
>      }
> +    if (xc_gnttab_set_max_grants(xendev->gnttabdev,
> +            2 * max_requests * BLKIF_MAX_SEGMENTS_PER_REQUEST) < 0) {
> +        xen_be_printf(xendev, 0, "xc_gnttab_set_max_grants failed: %s\n",
> +                      strerror(errno));
> +    }
>  }
>  
>  static int blk_init(struct XenDevice *xendev)
 

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