Re: [PATCH v3] iommu: Add driver for Renesas VMSA-compatible IPMMU

[Joerg Roedel <joro-zLv9SwRftAIdnm+yROfE0A@public.gmane.org>]

On Tue, May 13, 2014 at 11:04:10PM +0200, Laurent Pinchart wrote:
> > Isn't this the same as ipmmu_tlb_invalidate()?
> 
> ipmmu_tlb_invalidate() performs a read-update-write operation on the IMCTR 
> register to set the FLUSH bit without modifying the other bits, while this 
> function writes the FLUSH bit and sets all other bits (including the enable 
> bit) to zero. The difference is thus important.

Ah ok, I've seen that, but wasn't aware that it makes a difference.

> > Why not? This is something the IOMMU-API basically supports (multiple
> > devices behind different IOMMUs in the same domain). Can't you just use
> > the same page-table for different IOMMUs?
> 
> I might be able to (I'll need to check first though), but I don't really see 
> what the use cases for sharing a common page table between separate IOMMUs 
> would be. Could you please elaborate a bit ? Given that we're getting close to 
> the v3.16 merge window, would it be acceptable to fix (if needed) that as a 
> follow-up patch, as the driver is already usable as-is ?

The typical use-case is device-assignment in KVM, when you want to
assign multiple devices to the same guest it is desireable to only have
one domain with one page-table.
At least this is where this requirement comes from in the IOMMU-API, and
I like the IOMMU drivers to be consistent in their implementation of the
API.

I think given that otherwise the code looks good overall it is ok if you
do this as a follow-on patch to this one. I can put it in my tree for
the time being so that you can make a patch against the feature branch.


	Joerg