Linux Virtualization - Jul 2015 - [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 at redhat.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 at
redhat.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,
> >

On Thu, 2 Jul 2015 11:32:52 +0200
"Michael S. Tsirkin" <mst at redhat.com> wrote:
> 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 at redhat.com> wrote:
...
> > > 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.
Would something simple like this already do the job:

diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
--- a/drivers/vhost/Kconfig
+++ b/drivers/vhost/Kconfig
@@ -35,6 +35,7 @@ config VHOST
 
 config VHOST_CROSS_ENDIAN_LEGACY
 	bool "Cross-endian support for vhost"
+	depends on KVM_BOOK3S_64 || KVM_ARM_HOST
 	default n
 	---help---
 	  This option allows vhost to support guests with a different byte

?

If that looks acceptable, I can submit a proper patch if you like.

 Thomas

On Tue, Jul 07, 2015 at 06:36:53PM +0200, Thomas Huth
wrote:
> On Thu, 2 Jul 2015 11:32:52 +0200
> "Michael S. Tsirkin" <mst at redhat.com> wrote:
> 
> > 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 at redhat.com> wrote:
> ...
> > > > 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.
> 
> Would something simple like this already do the job:
> 
> diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
> --- a/drivers/vhost/Kconfig
> +++ b/drivers/vhost/Kconfig
> @@ -35,6 +35,7 @@ config VHOST
>  
>  config VHOST_CROSS_ENDIAN_LEGACY
>  	bool "Cross-endian support for vhost"
> +	depends on KVM_BOOK3S_64 || KVM_ARM_HOST
>  	default n
>  	---help---
>  	  This option allows vhost to support guests with a different byte
> 
> ?
Do all ARM hosts support this dynamic endian-ness?
> If that looks acceptable, I can submit a proper patch if you like.
> 
>  Thomas
I think I prefer some kind of symbol defined by these arches,
so I don't get to maintain an arch list in vhost.

-- 
MST