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

On 18/03/14 15:54, Konrad Rzeszutek Wilk wrote:
On Tue, Mar 18, 2014 at 03:42:56PM +0000, Zoltan Kiss wrote:
On 18/03/14 13:38, Konrad Rzeszutek Wilk wrote:
On Tue, Mar 18, 2014 at 01:21:18PM +0000, Zoltan Kiss wrote:

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
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

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.

I was thinking it was triggered on your 'return value'.

In this case I would say just use asm goto (jump labels) and maybe have
it turned on via debugfs? That would make all of this based on the
I would prefer avoiding the introduction of a new config parameter, if possible. A module parameter rather, but as I'm lazy, and the importance of this checking is not too high, the best would be to bind this to some general "is debugging turned on?" option. If there would be a shorthand function "is_debugfs_mounted", that would be reasonable, but I couldn't find it.


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