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

On Thu, 2 Jul 2015 11:32:52 +0200
"Michael S. Tsirkin" [off-list ref] wrote:

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On Thu, Jul 02, 2015 at 11:12:56AM +0200, Greg Kurz wrote:

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On Thu, 2 Jul 2015 08:01:28 +0200
"Michael S. Tsirkin" [off-list ref] wrote:

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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

?