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

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Message-ID: <20150702071412-mutt-send-email-mst@redhat.com>
Date:	Thu, 2 Jul 2015 08:01:28 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	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>,
	gkurz@...ux.vnet.ibm.com, kraxel@...hat.com
Subject: Re: [PULL] virtio/vhost: cross endian support

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.

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

Thanks,

-- 
MST
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