linux-kernel - Re: [PATCH] vduse: add virtio

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Message-ID: <20250123200006.GB238888@fedora>
Date: Thu, 23 Jan 2025 15:00:06 -0500
From: Stefan Hajnoczi <stefanha@...hat.com>
To: Eugenio Perez Martin <eperezma@...hat.com>
Cc: Jason Wang <jasowang@...hat.com>, mst@...hat.com,
	virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org,
	Hanna Reitz <hreitz@...hat.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	German Maglione <gmaglione@...hat.com>
Subject: Re: [PATCH] vduse: add virtio_fs to allowed dev id

On Thu, Jan 23, 2025 at 08:26:57AM +0100, Eugenio Perez Martin wrote:
> On Thu, Jan 23, 2025 at 2:50 AM Jason Wang <jasowang@...hat.com> wrote:
> >
> > On Wed, Jan 22, 2025 at 11:49 PM Stefan Hajnoczi <stefanha@...hat.com> wrote:
> > >
> > > On Tue, Jan 21, 2025 at 11:33:46AM +0100, Eugenio Pérez wrote:
> > > > A VDUSE device that implements virtiofs device works fine just by
> > > > adding the device id to the whitelist.
> > >
> > > The virtiofs DAX Window feature might require extra work. It is not
> > > widely enabled though, so must virtiofs devices will work fine with just
> > > virtqueue and configuration space support.
> >
> > +1.
> >
> > The DAX Window may help to reduce the bounce buffer copying and fit
> > for the case where an AI application just accesses one single large
> > file.
> >
> > We need to consider implementing this via VDUSE.
> >
> > Thanks
> >
> >
> 
> I thought DAX can only be applied in the VM case. Is it possible to
> apply it using virtio_vdpa?

Hardware could implement the DAX Window feature. In virtio-pci it's
implemented as a range within a BAR on the PCI device.

> Is it possible to detect that the device is trying to use DAX so it
> fails the negotiation?

Yes, DAX is optional. It's only available when the device reports a
Shared Memory Region with ID 0:
https://github.com/oasis-tcs/virtio-spec/blob/master/device-types/fs/description.tex#L233

Just to let you know, the QEMU vhost-user implementation of the DAX
Window is not complete. In the past David Gilbert had out-of-tree
patches to enable it in QEMU.

The guest driver support is in Linux.

Stefan

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