Re: [Xen-devel] Grant unmap error checking in Dom0

[Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>]

On 18/03/14 13:38, Konrad Rzeszutek Wilk wrote:
> On Tue, Mar 18, 2014 at 01:21:18PM +0000, Zoltan Kiss wrote:
> > Hi,
> >
> > Just out of curiosity I've checked how Dom0 handles errors during
> > grant unmapping. Usually there is a BUG_ON(ret) for the return value
> > of gnttab_unmap_refs in blkback and netback, gntdev drops just a
> > WARN.
> > The return value can be non-zero only if Xen failed to copy the map
> > operations back and forth to the guest supplied memory, so it's
> > reasonable to crash there. However I'm wondering why gntdev is happy
> > with just a WARN.
> > Another thing, we don't check the status of the operations if the
> > return value is zero. We shouldn't normally do that, Xen logs info
> > messages in some cases, but not always (e.g. XSM or IOMMU problems).
> > For debugging purposes however it could be useful to have the
> > ability to turn on checking in Dom0. A quick and dirty way to do
> > this is to use printk_get_level to figure out if the loglevel is
> > e.g. KERN_NOTICE or lower, but I'm sure there is a better way to do
> > this :) It would be an overkill to introduce new config option, I'm
> > thinking a runtime parameter to check in an unlikely(), so it won't
> > cause performance penalty for normal operation. Any opinions on
> > that?
> One can always just have printk(KERN_DEBUG and if the user did not
> boot with 'debug' the messages go to /dev/null.
Checking through the unmap_ops array itself can take a considerable 
amount of cycles, I think it would be better to avoid that.
Zoli


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