netdev - Re: [PULL] virtio/vhost: cross endian support

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Message-ID: <20150702112809-mutt-send-email-mst@redhat.com>
Date:	Thu, 2 Jul 2015 11:32:52 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Greg Kurz <gkurz@...ux.vnet.ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	KVM list <kvm@...r.kernel.org>,
	virtualization <virtualization@...ts.linux-foundation.org>,
	Network Development <netdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Cornelia Huck <cornelia.huck@...ibm.com>, kraxel@...hat.com
Subject: Re: [PULL] virtio/vhost: cross endian support

On Thu, Jul 02, 2015 at 11:12:56AM +0200, Greg Kurz wrote:
> On Thu, 2 Jul 2015 08:01:28 +0200
> "Michael S. Tsirkin" <mst@...hat.com> wrote:
> 
> > On Wed, Jul 01, 2015 at 12:02:50PM -0700, Linus Torvalds wrote:
> > > On Wed, Jul 1, 2015 at 2:31 AM, Michael S. Tsirkin <mst@...hat.com> wrote:
> > > > virtio/vhost: cross endian support
> > > 
> > > Ugh. Does this really have to be dynamic?
> > > 
> > > Can't virtio do the sane thing, and just use a _fixed_ endianness?
> > > 
> > > Doing a unconditional byte swap is faster and simpler than the crazy
> > > conditionals. That's true regardless of endianness, but gets to be
> > > even more so if the fixed endianness is little-endian, since BE is
> > > not-so-slowly fading from the world.
> > > 
> > >                Linus
> > 
> > Yea, well - support for legacy BE guests on the new LE hosts is
> > exactly the motivation for this.
> > 
> > I dislike it too, but there are two redeeming properties that
> > made me merge this:
> > 
> > 1.  It's a trivial amount of code: since we wrap host/guest accesses
> >     anyway, almost all of it is well hidden from drivers.
> > 
> > 2.  Sane platforms would never set flags like VHOST_CROSS_ENDIAN_LEGACY -
> >     and when it's clear, there's zero overhead (as some point it was
> >     tested by compiling with and without the patches, got the same
> >     stripped binary).
> > 
> > Maybe we could create a Kconfig symbol to enforce point (2): prevent
> > people from enabling it e.g. on x86. I will look into this - but it can
> > be done by a patch on top, so I think this can be merged as is.
> > 
> 
> This cross-endian *oddity* is targeting PowerPC book3s_64 processors... I
> am not aware of any other users. Maybe create a symbol that would
> be only selected by PPC_BOOK3S_64 ?

I think some ARM systems are trying to support cross-endian
configurations as well.

Besides that, yes, this is more or less what I had in mind.

> 
> > Or do you know of someone using kernel with all config options enabled
> > undiscriminately?
> > 
> > Thanks,
> > 
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