Re: Grant unmap error checking in Dom0

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

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:
> >>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.


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
CONFIG_XEN_DEBUGFS?


> 
> Zoli
>