Re: [RFC 0/4] Enable virtio-fs on s390x

From: Halil Pasic <pasic@linux.ibm.com>
To: "Daniel P. Berrangé" <berrange@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	"Cornelia Huck" <cohuck@redhat.com>,
	qemu-devel@nongnu.org, "Marc Hartmayer" <mhartmay@linux.ibm.com>,
	"Stefan Hajnoczi" <stefanha@redhat.com>,
	"Marc-André Lureau" <marcandre.lureau@redhat.com>
Subject: Re: [RFC 0/4] Enable virtio-fs on s390x
Date: Thu, 25 Jun 2020 14:21:02 +0200	[thread overview]
Message-ID: <20200625142102.1f82ae4d.pasic@linux.ibm.com> (raw)
In-Reply-To: <20200625101935.GF1009994@redhat.com>

On Thu, 25 Jun 2020 11:19:35 +0100
Daniel P. Berrangé <berrange@redhat.com> wrote:

> On Thu, Jun 25, 2020 at 12:04:26PM +0200, Marc Hartmayer wrote:
> > This RFC is about enabling virtio-fs on s390x. For that we need
> >  + some shim code (first patch), and we need
> >  + libvhost-user to deal with virtio endiannes as mandated by the spec.
> >  
> > The second part is trickier, because unlike QEMU we are not certain
> > about the guest's native endianness, which is needed to handle the
> > legacy-interface appropriately. In fact, this is the reason why just
> > RFC.
> > 
> > One of the open questions is whether to build separate versions, one
> > for guest little endian and one for guest big endian, or do we want
> > something like a command line option? (Digression on the libvirt
> > modeling)
> 
> When you talk about  big vs little endian, are you referring to TCG
> scenarios with mixed host/guest arch, or arches which can support
> either endianess, or both ? i guess it doesn't matter actually, as
> I think the latter forces a specific answer.
> 

Our primary concern is big endian cpu and little endian virtio because
virto 1.0 or better. But since we talk about a lib here, everything is
possible.

> Considering that some architectures allow the guest OS to flip between
> big & little endian as they boot, libvirt cannot know what endianess
> the guest is using when it launches virtiofsd. It thus cannot pick
> between two different endianness builds of virtiofsd automatically.
> This would force the user to tell libvirt what arch the guest is using
> at the time they define the guest. This is an undesirable restriction
> for use cases where the admin of the guest OS has no direct control
> over the host config.
> 
> IOW, I think the only practical answer is to have a single binary that
> automagically does the right thing at runtime according to guest
> endianess that currently is in use.
> 

The problem is that AFAIK there is no way to figure out what is the right
thing at the moment. I guess QEMU should now, but not virtiofsd or
whatever userspace virtio device. But I will double check if there is
something in the protocol addressing this.

Regards,
Halil