linux-kernel - Re: [PATCH v2 7/7] iommu/s390: flush queued IOVAs on RPCIT out of resource indication

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Date:   Fri, 02 Dec 2022 16:12:50 +0100
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        Gerd Bayer <gbayer@...ux.ibm.com>, iommu@...ts.linux.dev,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Wenjia Zhang <wenjia@...ux.ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>,
        linux-s390@...r.kernel.org, borntraeger@...ux.ibm.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, agordeev@...ux.ibm.com,
        svens@...ux.ibm.com, linux-kernel@...r.kernel.org,
        Julian Ruess <julianr@...ux.ibm.com>
Subject: Re: [PATCH v2 7/7] iommu/s390: flush queued IOVAs on RPCIT out of
 resource indication

On Fri, 2022-12-02 at 10:42 -0400, Jason Gunthorpe wrote:
> On Fri, Dec 02, 2022 at 03:29:50PM +0100, Niklas Schnelle wrote:
> 
> > @Robin @Joerg, if you are open to changing .iotlb_sync_map such that it
> > can return and error and then failing the mapping operation I think
> > this is a great approach. One advantage over the previous approach of
> > flushing the queue isthat this should work for the pure IOMMU API too.
> 
> I think it is pretty important that the "pure" IOMMU API work with
> whatever your solution its, the iommu_domain implementation in a
> driver should not be sensitive to if the dma-iommu.c is operating the
> domain or something else.
> 
> Jason

Agree. At the moment, i.e. with current mainline and even with the
IOMMU improvements in Joerg's queue, the IOMMU APIdoesn't work when the
hypervisor returns out-of-resource from the IOTLB flush (RPCIT). This
is currently only handled correctly in the arch/s390/pci/pci_dma.c DMA
API implementation. I think both handling this with a hidden/inlined
.iotlb_sync_map in s390_iommu_map_pages() or with an enhanced
.iotlb_sync_map that then must be able to return an error will fix this
shortcoming.  The latter would be something like this commit I'm
currently testing with:

https://git.kernel.org/pub/scm/linux/kernel/git/niks/linux.git/commit/?h=dma_iommu_v3&id=a2aecd821fe3c5e2549946a68d8b07e05b288a9b

Thanks,
Niklas