linux-kernel - Re: VFIO MSIX Query

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Date:	Mon, 9 May 2016 09:20:15 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Nitin Saxena <nitin.lnx@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: VFIO MSIX Query

On Sun, 8 May 2016 20:45:10 +0530
Nitin Saxena <nitin.lnx@...il.com> wrote:

> Hi Alex,
> 
> Thanks for your answer. I got the point that all access to device must be
> VFIO.
> I have few more questions.
> 
> >> When the device is initially opened via the VFIO_GET_DEVICE_FD ioctl, we  
> go through an initialization sequence on the device, including a device
> reset
> Does this mean only one process (primary in multiprocess app) can only
> perform VFIO_GROUP_GET_DEVICE_FD ioctl ? which I doubt as I saw in dpdk
> that each process (regardless primary or not) does VFIO_GROUP_GET_DEVICE_FD
> ioctl. Kindly confirm.

Users can call GET_DEVICE_FD multiple times, but note that the group
file descriptor can only be opened once, so you'd need to pass
device file descriptors off to other threads.

> >> Use VFIO for all access to the device or don't use VFIO at all.  
> I will map PCI BARS via VFIO but what about hugepages for each process.
> Since I am using VFIO_NOIOMMU do I need to call VFIO_IOMMU_MAP_DMA ioctl
> for each hugepage of each process? I think this is not needed in my case.
> Please confirm.

No-IOMMU mode does not have MAP_DMA or UNMAP_DMA ioctls.  If you want
to support a safe operating mode, with an IOMMU providing DMA
isolation, then yes, you would need to map DMA targets for the device
using those ioctls for the type1 IOMMU backend.

> Can you please also confirm about the patch I attached. Is there any thing
> missing in the patch that I needed to incorporate from kernel 4.5 to make
> VFO NOIOMMU work? Patch

I can't confirm or deny whether the backport is accurate.  Thanks,

Alex