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Subject: Re: [PATCH v2] vfio iommu: Add dma available capability
From:   Matthew Rosato <mjrosato@linux.ibm.com>
To:     alex.williamson@redhat.com, cohuck@redhat.com
Cc:     pmorel@linux.ibm.com, schnelle@linux.ibm.com, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
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Date:   Mon, 14 Sep 2020 18:32:41 -0400
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On 9/14/20 6:25 PM, Matthew Rosato wrote:
> Commit 492855939bdb ("vfio/type1: Limit DMA mappings per container") added
> a limit to the number of concurrent DMA requests for a vfio container.
> However, lazy unmapping in s390 can in fact cause quite a large number of
> outstanding DMA requests to build up prior to being purged, potentially
> the entire guest DMA space.  This results in unexpected 'VFIO_MAP_DMA
> failed: No space left on device' conditions seen in QEMU.
> 
> This patch proposes to provide the remaining number of allowable DMA
> requests via the VFIO_IOMMU_GET_INFO ioctl as a new capability.  A
> subsequent patchset to QEMU would collect this information and use it in
> s390 PCI support to tap the guest on the shoulder before overrunning the
> vfio limit.

Link to associated v2 QEMU patchset:
https://lists.gnu.org/archive/html/qemu-devel/2020-09/msg05067.html

> 
> Changes from v1:
> - Report dma_avail instead of the limit, which might not have been accurate
>    anyway
> - Text/naming changes throughout due to the above
> 
> Matthew Rosato (1):
>    vfio iommu: Add dma available capability
> 
>   drivers/vfio/vfio_iommu_type1.c | 17 +++++++++++++++++
>   include/uapi/linux/vfio.h       | 16 ++++++++++++++++
>   2 files changed, 33 insertions(+)
>