Re: [PATCH v3 5/5] intel_iommu: Optimize out some unnecessary UNMAP calls

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Jun 08, 2023 at 11:40:55AM -0400, Peter Xu wrote:

> > But if there is the proper locks to prevent a map/unmap race, then
> > there should also be the proper locks to check that there is no map in
> > the first place and avoid the kernel call..
> 
> The problem is IIRC guest iommu driver can do smart things like batching
> invalidations, it means when QEMU gets it from the guest OS it may already
> not matching one mapped objects.

qemu has to fix it. The kernel API is object based, not paged
based. You cannot unmap partions of a prior mapping.

I assume for this kind of emulation it is doing 4k objects because
it has no idea what size of mapping the client will use?

> We can definitely lookup every single object and explicitly unmap, but it
> loses partial of the point of batching that guest OS does.  

You don't need every single object, but it would be faster to check
where things are mapped and then call the kernel correctly instead of
trying to iterate with the unmapped reults.

Jason