linux-kernel - Re: [PATCH v4 04/42] virtio: disable virtio 1.0 in transports

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Date:	Thu, 27 Nov 2014 12:54:34 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Cornelia Huck <cornelia.huck@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
	rusty@....ibm.com, nab@...ux-iscsi.org, pbonzini@...hat.com,
	Rusty Russell <rusty@...tcorp.com.au>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	linux390@...ibm.com, Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Pawel Moll <pawel.moll@....com>,
	Ohad Ben-Cohen <ohad@...ery.com>,
	Sudeep Dutt <sudeep.dutt@...el.com>,
	Ashutosh Dixit <ashutosh.dixit@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Nikhil Rao <nikhil.rao@...el.com>,
	Siva Yerramreddy <yshivakrishna@...il.com>,
	lguest@...ts.ozlabs.org, linux-s390@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v4 04/42] virtio: disable virtio 1.0 in transports

On Wed, Nov 26, 2014 at 10:09:54AM +0100, Cornelia Huck wrote:
> On Tue, 25 Nov 2014 23:20:11 +0200
> "Michael S. Tsirkin" <mst@...hat.com> wrote:
> 
> > On Tue, Nov 25, 2014 at 06:29:42PM +0100, Cornelia Huck wrote:
> > > On Tue, 25 Nov 2014 18:41:35 +0200
> > > "Michael S. Tsirkin" <mst@...hat.com> wrote:
> > > 
> > > > disable virtio 1.0 in transports that don't
> > > > support it yet.
> > > 
> > > I'd prefer if you disabled it for _every_ transport in this patch,
> > > until the needed infrastructure is in place. Else this is a bit
> > > confusing.
> > 
> > Well the only transports left are pci and rpoc, and these only
> > read the low 32 bit of the features from the device -
> > so there's nothing to clear.
> > 
> > E.g. the following would be even more confusing, would it not:
> > 
> > 	u32 features;
> > 	....
> > 	features &= ~BIT_ULL(VIRTIO_F_VERSION_1);
> > 
> > Agree?
> 
> Maybe you should tweak the patch description a bit and mention that you
> only disable virtio 1.0 for transports where it is actually needed?
> 
> (...)

Yes, I did that now.

> > > FWIW, as negotiating a revision >= 1 is a pre-req for virtio 1.0
> > > support on ccw, virtio 1.0 is already implicitly disabled.
> > 
> > Ah, you mean device guarantees that VIRTIO_F_VERSION_1 isn't set
> > if guest sets revision to 0?
> 
> Yes, the bit will not be offered if the revision is 0 or has not been
> set at all.
> 
> > In that case it's probably best to drop this from both ccw
> > devices.
> 
> There's only one ccw transport :)
> 
> The old s390 virtio transport in kvm_virtio.c is not part of virtio 1.0.

It might or might not be a good idea to add code in kvm_virtio.c
blacklisting VIRTIO_F_VERSION_1, just in case there's a buggy device
that sets it.
As correct devices won't set it, I don't think we need to
worry about it too much. We can make it a patch on top later
if we want.

-- 
MST
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