Re: [PATCH v4 8/8] macvtap/tun: add VNET_BE flag
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From: Michael S. Tsirkin <hidden>
Date: 2015-04-21 18:30:40
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On Tue, Apr 21, 2015 at 06:22:20PM +0200, Greg Kurz wrote:
On Tue, 21 Apr 2015 16:06:33 +0200 "Michael S. Tsirkin" [off-list ref] wrote:quoted
On Fri, Apr 10, 2015 at 12:20:21PM +0200, Greg Kurz wrote:quoted
The VNET_LE flag was introduced to fix accesses to virtio 1.0 headers that are always little-endian. It can also be used to handle the special case of a legacy little-endian device implemented by a big-endian host. Let's add a flag and ioctls for big-endian devices as well. If both flags are set, little-endian wins. Since this is isn't a common usecase, the feature is controlled by a kernel config option (not set by default). Both macvtap and tun are covered by this patch since they share the same API with userland. Signed-off-by: Greg Kurz <redacted> --- drivers/net/Kconfig | 12 ++++++++ drivers/net/macvtap.c | 60 +++++++++++++++++++++++++++++++++++++++++- drivers/net/tun.c | 62 ++++++++++++++++++++++++++++++++++++++++++- include/uapi/linux/if_tun.h | 2 + 4 files changed, 134 insertions(+), 2 deletions(-)diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig index df51d60..f0e23a0 100644 --- a/drivers/net/Kconfig +++ b/drivers/net/Kconfig@@ -244,6 +244,18 @@ config TUN If you don't know what to use this for, you don't need it. +config TUN_VNET_BE + bool "Support for big-endian vnet headers" + default n + ---help--- + This option allows TUN/TAP and MACVTAP device drivers to parse + vnet headers that are in big-endian byte order. It is useful + when the headers come from a big-endian legacy virtio driver and + the host is little-endian. + + Unless you have a little-endian system hosting a big-endian virtual + machine with a virtio NIC, you should say N. +should mention cross-endian, not big-endian, right?The current TUN_VNET_LE related code is already doing cross-endian: without this patch, one can already run a LE guest on a BE host... wouldn't it be confusing to mention cross-endian only when the guest is BE ?
Hmm I think no - LE is also useful for virtio 1 - this is what it was intended for after all.
What about having a completely distinct implementation for cross-endian that don't reuse the existing code and defines then ?
I think implementation and interface are fine, just the documentation can be improved a bit. How about: "Support for cross-endian vnet headers on little-endian kernels". Accordingly CONFIG_TUN_VNET_CROSS_LE ? -- MST