linux-kernel - Re: [PATCH v3] vfio iommu: Add dma available capability

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Message-ID: <4437b2a0-c2db-4210-305a-3397bcc18050@linux.ibm.com>
Date:   Tue, 15 Sep 2020 15:18:12 -0400
From:   Matthew Rosato <mjrosato@...ux.ibm.com>
To:     alex.williamson@...hat.com, cohuck@...hat.com
Cc:     pmorel@...ux.ibm.com, schnelle@...ux.ibm.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] vfio iommu: Add dma available capability

On 9/15/20 3:05 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 latest QEMU patchset:
https://lists.gnu.org/archive/html/qemu-devel/2020-09/msg05505.html

> 
> Changes from v2:
> - Typos / fixed stale comment block
> 
> Matthew Rosato (1):
>    vfio iommu: Add dma available capability
> 
>   drivers/vfio/vfio_iommu_type1.c | 17 +++++++++++++++++
>   include/uapi/linux/vfio.h       | 15 +++++++++++++++
>   2 files changed, 32 insertions(+)
>