RE: [PATCH v2] vfio: do not coalesce DMA mappings

["Gupta, Nipun" <Nipun.Gupta@amd.com>]

[AMD Official Use Only - General]

Hi David,

I agree that change is not straightforward to review, but it should not cause any functional issue as we are still creating all the memory mappings, but one by one for each segment.
For hot plug case this causes issue as mentioned, that VFIO does not allow unmap of the individual segments in case mapping was created of a single coalesced segment. 

But yes, I am not sure why this code was added, which Anatoly may have more understanding on.

Anatoly,

Can you please provide your feedback on this change?

Thanks,
Nipun

> -----Original Message-----
> From: David Marchand <david.marchand@redhat.com>
> Sent: Thursday, February 2, 2023 4:19 PM
> To: anatoly.burakov@intel.com; Gupta, Nipun <Nipun.Gupta@amd.com>
> Cc: dev@dpdk.org; thomas@monjalon.net; Yigit, Ferruh
> <Ferruh.Yigit@amd.com>; Agarwal, Nikhil <nikhil.agarwal@amd.com>
> Subject: Re: [PATCH v2] vfio: do not coalesce DMA mappings
> 
> Caution: This message originated from an External Source. Use proper caution
> when opening attachments, clicking links, or responding.
> 
> 
> Hi,
> 
> On Wed, Jan 4, 2023 at 6:19 AM Nipun Gupta <nipun.gupta@amd.com> wrote:
> >
> > At the cleanup time when dma unmap is done, linux kernel
> > does not allow unmap of individual segments which were
> > coalesced together while creating the DMA map for type1 IOMMU
> > mappings. So, this change updates the mapping of the memory
> > segments(hugepages) on a per-page basis.
> >
> > Signed-off-by: Nipun Gupta <nipun.gupta@amd.com>
> > Signed-off-by: Nikhil Agarwal <nikhil.agarwal@amd.com>
> 
> This change is scary.
> 
> I won't take it without a review from the maintainer.
> Anatoly, can you have a look?
> 
> 
> Thanks.
> --
> David Marchand