| Message ID | 20150701113150-mutt-send-email-mst@redhat.com |
|---|---|
| State | Not Applicable, archived |
| Delegated to: | David Miller |
| Headers | show |
On Wed, Jul 1, 2015 at 2:31 AM, Michael S. Tsirkin <mst@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
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On Wed, Jul 1, 2015 at 12:02 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote: > > Doing a unconditional byte swap is faster and simpler than the crazy > conditionals. Unconditional endianness not only makes for simpler and faster code, it also ends up being easier to debug and add things like type annotations for sparse. Linus -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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@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. Or do you know of someone using kernel with all config options enabled undiscriminately? Thanks,
On Wed, Jul 01, 2015 at 12:03:59PM -0700, Linus Torvalds wrote: > On Wed, Jul 1, 2015 at 12:02 PM, Linus Torvalds > <torvalds@linux-foundation.org> wrote: > > > > Doing a unconditional byte swap is faster and simpler than the crazy > > conditionals. > > Unconditional endianness not only makes for simpler and faster code, > it also ends up being easier to debug and add things like type > annotations for sparse. > > Linus At least this last one is well covered by these patches: this uses separate sparse types so all accesses are statically verified by sparse to use the correct accessor.
On Thu, 2 Jul 2015 08:01:28 +0200 "Michael S. Tsirkin" <mst@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@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 ? > Or do you know of someone using kernel with all config options enabled > undiscriminately? > > Thanks, > -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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@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@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, > > -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, Jul 02, 2015 at 08:01:28AM +0200, Michael S. Tsirkin 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@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. Linus, could you please clarify whether making the feature depend on the cross-endian guest support would address your comment, and whether you think this can be merged as is, and the dependency added after -rc1? > Or do you know of someone using kernel with all config options enabled > undiscriminately? > > Thanks, > > -- > MST -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Fri, Jul 3, 2015 at 12:59 AM, Michael S. Tsirkin <mst@redhat.com> wrote: > > Linus, could you please clarify whether making the > feature depend on the cross-endian guest support > would address your comment, and whether you think > this can be merged as is, and the dependency added > after -rc1? I'll take it. I still think dynamic byte order is a fundamental (and much too common) mistake, but whatever. Linus -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
The following changes since commit 8a7b19d8b542b87bccc3eaaf81dcc90a5ca48aea: include/uapi/linux/virtio_balloon.h: include linux/virtio_types.h (2015-06-01 15:46:54 +0200) are available in the git repository at: git://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git tags/for_linus for you to fetch changes up to 59a5b0f7bf74f88da6670bcbf924d8cc1e75b1ee: virtio-pci: alloc only resources actually used. (2015-06-24 08:15:09 +0200) ---------------------------------------------------------------- virtio/vhost: cross endian support I have just queued some more bugfix patches today but none fix regressions and none are related to these ones, so it looks like a good time for a merge for -rc1. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> ---------------------------------------------------------------- Gerd Hoffmann (1): virtio-pci: alloc only resources actually used. Greg Kurz (8): virtio: introduce virtio_is_little_endian() helper tun: add tun_is_little_endian() helper macvtap: introduce macvtap_is_little_endian() helper vringh: introduce vringh_is_little_endian() helper vhost: introduce vhost_is_little_endian() helper virtio: add explicit big-endian support to memory accessors vhost: cross-endian support for legacy devices macvtap/tun: cross-endian support for little-endian hosts drivers/vhost/vhost.h | 25 ++++++++--- drivers/virtio/virtio_pci_common.h | 2 + include/linux/virtio_byteorder.h | 24 ++++++----- include/linux/virtio_config.h | 18 +++++--- include/linux/vringh.h | 18 +++++--- include/uapi/linux/if_tun.h | 6 +++ include/uapi/linux/vhost.h | 14 +++++++ drivers/net/macvtap.c | 65 ++++++++++++++++++++++++++++- drivers/net/tun.c | 67 +++++++++++++++++++++++++++++- drivers/vhost/vhost.c | 85 +++++++++++++++++++++++++++++++++++++- drivers/virtio/virtio_pci_common.c | 7 ---- drivers/virtio/virtio_pci_legacy.c | 13 +++++- drivers/virtio/virtio_pci_modern.c | 24 ++++++++--- drivers/net/Kconfig | 14 +++++++ drivers/vhost/Kconfig | 15 +++++++ 15 files changed, 350 insertions(+), 47 deletions(-) -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html