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

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Message-ID: <20141126100954.3811e594.cornelia.huck@de.ibm.com>
Date:	Wed, 26 Nov 2014 10:09:54 +0100
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.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 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?

(...)

> > 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.

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