Re: [Qemu-devel] [PATCH v7 1/8] virtio: endian-ambivalent targets using legacy virtio

[Michael S. Tsirkin]

On Mon, Apr 14, 2014 at 02:40:04PM +0200, Alexander Graf wrote:
> 
> On 14.04.14 14:37, Michael S. Tsirkin wrote:
> >On Mon, Apr 14, 2014 at 02:29:20PM +0200, Alexander Graf wrote:
> >>On 14.04.14 14:24, Michael S. Tsirkin wrote:
> >>>On Mon, Apr 14, 2014 at 02:16:03PM +0200, Alexander Graf wrote:
> >>>>On 14.04.14 13:58, Greg Kurz wrote:
> >>>>>From: Rusty Russell <address@hidden>
> >>>>>
> >>>>>virtio data structures are defined as "target endian", which assumes
> >>>>>that's a fixed value.  In fact, that actually means it's 
> >>>>>platform-specific.
> >>>>>The OASIS virtio 1.0 spec will fix this, by making it all little endian.
> >>>>>
> >>>>>We introduce memory accessors to be used accross the virtio code where
> >>>>>needed. These accessors should support both legacy and 1.0 devices.
> >>>>>A good way to do it is to introduce a per-device property to store the
> >>>>>endianness. We choose to set this flag at device reset time because it
> >>>>>is reasonnable to assume the endianness won't change unless we reboot or
> >>>>>kexec another kernel. And it is also reasonnable to assume the new kernel
> >>>>>will reset the devices before using them (otherwise it will break).
> >>>>>
> >>>>>We reuse the virtio_is_big_endian() helper since it provides the right
> >>>>>value for legacy devices with most of the targets, that have fixed
> >>>>>endianness. It can then be overriden to support endian-ambivalent 
> >>>>>targets.
> >>>>>
> >>>>>To support migration, we need to set the flag in virtio_load() as well.
> >>>>>
> >>>>>(a) One solution would be to add it to the stream, but it have some
> >>>>>     drawbacks:
> >>>>>- since this only affects a few targets, the field should be put into a
> >>>>>   subsection
> >>>>>- virtio migration code should be ported to vmstate to be able to 
> >>>>>introduce
> >>>>>   such a subsection
> >>>>>
> >>>>>(b) If we assume the following to be true:
> >>>>>- target endianness falls under some cpu state
> >>>>>- cpu state is always restored before virtio devices state because they
> >>>>>   get initialized in this order in main().
> >>>>>Then an alternative is to rely on virtio_is_big_endian() again at
> >>>>>load time. No need to mess around with the migration stream in this case.
> >>>>>
> >>>>>This patch implements (b).
> >>>>>
> >>>>>Note that the tswap helpers are implemented in virtio.c so that
> >>>>>virtio-access.h stays platform independant. Most of the virtio code
> >>>>>will be buildable under common-obj instead of obj then, and spare
> >>>>>some cycles when building for multiple targets.
> >>>>>
> >>>>>Signed-off-by: Rusty Russell <address@hidden>
> >>>>>[ ldq_phys() API change,
> >>>>>   relicensed virtio-access.h to GPLv2+ on Rusty's request,
> >>>>>   introduce a per-device is_big_endian flag (supersedes needs_byteswap 
> >>>>> global)
> >>>>>   add VirtIODevice * arg to virtio helpers,
> >>>>>   use the existing virtio_is_big_endian() helper,
> >>>>>   virtio-pci: use the device is_big_endian flag,
> >>>>>   introduce virtio tswap16 and tswap64 helpers,
> >>>>>   move calls to tswap* out of virtio-access.h to make it platform 
> >>>>> independant,
> >>>>>   migration support,
> >>>>>   Greg Kurz <address@hidden> ]
> >>>>>Cc: Cédric Le Goater <address@hidden>
> >>>>>Signed-off-by: Greg Kurz <address@hidden>
> >>>>>---
> >>>>>
> >>>>>Changes since v6:
> >>>>>- merge the virtio_needs_byteswap() helper from v6 and existing
> >>>>>   virtio_is_big_endian()
> >>>>>- virtio-pci: now supports endianness changes
> >>>>>- virtio-access.h fixes (target independant)
> >>>>>
> >>>>>  exec.c                            |    2 -
> >>>>>  hw/virtio/Makefile.objs           |    2 -
> >>>>>  hw/virtio/virtio-pci.c            |   11 +--
> >>>>>  hw/virtio/virtio.c                |   35 +++++++++
> >>>>>  include/hw/virtio/virtio-access.h |  138 
> >>>>> +++++++++++++++++++++++++++++++++++++
> >>>>>  include/hw/virtio/virtio.h        |    2 +
> >>>>>  vl.c                              |    4 +
> >>>>>  7 files changed, 185 insertions(+), 9 deletions(-)
> >>>>>  create mode 100644 include/hw/virtio/virtio-access.h
> >>>>>
> >>>>>diff --git a/exec.c b/exec.c
> >>>>>index 91513c6..e6777d0 100644
> >>>>>--- a/exec.c
> >>>>>+++ b/exec.c
> >>>>>@@ -42,6 +42,7 @@
> >>>>>  #else /* !CONFIG_USER_ONLY */
> >>>>>  #include "sysemu/xen-mapcache.h"
> >>>>>  #include "trace.h"
> >>>>>+#include "hw/virtio/virtio.h"
> >>>>>  #endif
> >>>>>  #include "exec/cpu-all.h"
> >>>>>@@ -2745,7 +2746,6 @@ int cpu_memory_rw_debug(CPUState *cpu, 
> >>>>>target_ulong addr,
> >>>>>   * A helper function for the _utterly broken_ virtio device model to 
> >>>>> find out if
> >>>>>   * it's running on a big endian machine. Don't do this at home kids!
> >>>>>   */
> >>>>>-bool virtio_is_big_endian(void);
> >>>>>  bool virtio_is_big_endian(void)
> >>>>>  {
> >>>>>  #if defined(TARGET_WORDS_BIGENDIAN)
> >>>>>diff --git a/hw/virtio/Makefile.objs b/hw/virtio/Makefile.objs
> >>>>>index 1ba53d9..68c3064 100644
> >>>>>--- a/hw/virtio/Makefile.objs
> >>>>>+++ b/hw/virtio/Makefile.objs
> >>>>>@@ -4,5 +4,5 @@ common-obj-y += virtio-bus.o
> >>>>>  common-obj-y += virtio-mmio.o
> >>>>>  common-obj-$(CONFIG_VIRTIO_BLK_DATA_PLANE) += dataplane/
> >>>>>-obj-y += virtio.o virtio-balloon.o
> >>>>>+obj-y += virtio.o virtio-balloon.o
> >>>>>  obj-$(CONFIG_LINUX) += vhost.o
> >>>>>diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> >>>>>index ce97514..82a1689 100644
> >>>>>--- a/hw/virtio/virtio-pci.c
> >>>>>+++ b/hw/virtio/virtio-pci.c
> >>>>>@@ -89,9 +89,6 @@
> >>>>>  /* Flags track per-device state like workarounds for quirks in older 
> >>>>> guests. */
> >>>>>  #define VIRTIO_PCI_FLAG_BUS_MASTER_BUG  (1 << 0)
> >>>>>-/* HACK for virtio to determine if it's running a big endian guest */
> >>>>>-bool virtio_is_big_endian(void);
> >>>>>-
> >>>>>  static void virtio_pci_bus_new(VirtioBusState *bus, size_t bus_size,
> >>>>>                                 VirtIOPCIProxy *dev);
> >>>>>@@ -409,13 +406,13 @@ static uint64_t virtio_pci_config_read(void 
> >>>>>*opaque, hwaddr addr,
> >>>>>          break;
> >>>>>      case 2:
> >>>>>          val = virtio_config_readw(vdev, addr);
> >>>>>-        if (virtio_is_big_endian()) {
> >>>>>+        if (vdev->is_big_endian) {
> >>>>>              val = bswap16(val);
> >>>>>          }
> >>>>>          break;
> >>>>>      case 4:
> >>>>>          val = virtio_config_readl(vdev, addr);
> >>>>>-        if (virtio_is_big_endian()) {
> >>>>>+        if (vdev->is_big_endian) {
> >>>>>              val = bswap32(val);
> >>>>>          }
> >>>>>          break;
> >>>>>@@ -443,13 +440,13 @@ static void virtio_pci_config_write(void *opaque, 
> >>>>>hwaddr addr,
> >>>>>          virtio_config_writeb(vdev, addr, val);
> >>>>>          break;
> >>>>>      case 2:
> >>>>>-        if (virtio_is_big_endian()) {
> >>>>>+        if (vdev->is_big_endian) {
> >>>>>              val = bswap16(val);
> >>>>>          }
> >>>>>          virtio_config_writew(vdev, addr, val);
> >>>>>          break;
> >>>>>      case 4:
> >>>>>-        if (virtio_is_big_endian()) {
> >>>>>+        if (vdev->is_big_endian) {
> >>>>>              val = bswap32(val);
> >>>>>          }
> >>>>>          virtio_config_writel(vdev, addr, val);
> >>>>>diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> >>>>>index aeabf3a..bb646f0 100644
> >>>>>--- a/hw/virtio/virtio.c
> >>>>>+++ b/hw/virtio/virtio.c
> >>>>>@@ -19,6 +19,7 @@
> >>>>>  #include "hw/virtio/virtio.h"
> >>>>>  #include "qemu/atomic.h"
> >>>>>  #include "hw/virtio/virtio-bus.h"
> >>>>>+#include "hw/virtio/virtio-access.h"
> >>>>>  /*
> >>>>>   * The alignment to use between consumer and producer parts of vring.
> >>>>>@@ -546,6 +547,8 @@ void virtio_reset(void *opaque)
> >>>>>      virtio_set_status(vdev, 0);
> >>>>>+    vdev->is_big_endian = virtio_is_big_endian();
> >>>>>+
> >>>>>      if (k->reset) {
> >>>>>          k->reset(vdev);
> >>>>>      }
> >>>>>@@ -897,6 +900,11 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f)
> >>>>>      BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
> >>>>>      VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
> >>>>>+    /* NOTE: we assume that endianness is a cpu state AND
> >>>>>+     * cpu state is restored before virtio devices.
> >>>>>+     */
> >>>>>+    vdev->is_big_endian = virtio_is_big_endian();
> >>>>>+
> >>>>>      if (k->load_config) {
> >>>>>          ret = k->load_config(qbus->parent, f);
> >>>>>          if (ret)
> >>>>>@@ -1153,6 +1161,33 @@ void 
> >>>>>virtio_device_set_child_bus_name(VirtIODevice *vdev, char *bus_name)
> >>>>>      }
> >>>>>  }
> >>>>>+uint16_t virtio_tswap16(VirtIODevice *vdev, uint16_t s)
> >>>>>+{
> >>>>>+    if (vdev->is_big_endian) {
> >>>>>+        return tswap16(s);
> >>>>>+    } else {
> >>>>>+        return bswap16(tswap16(s));
> >>>>>+    }
> >>>>This looks pretty bogus. When virtio wants to do "tswap" what it
> >>>>means is "give me a host endianness value in virtio endianness". How
> >>>>about something like
> >>>>
> >>>>#ifdef HOST_WORDS_BIGENDIAN
> >>>>   return vdev->is_big_endian ? s : bswap16(s);
> >>>>#else
> >>>>   return vdev->is_big_endian ? bswap16(s) : s;
> >>>>#endif
> >>>>
> >>>Actually why doesn't this call virtio_is_big_endian?
> >>>As it is, we get extra branches even if target endian-ness
> >>>is fixed.
> >>Because virtio_is_big_endian() returns the default endianness, not
> >>the runtime endianness of a virtio device.
> 
> In fact, we should probably rename it accordingly.
> 
> >>
> >>>>That should work pretty well inline as well, so you don't need to
> >>>>compile virtio.c as target dependent.
> >>>>
> >>>>
> >>>>Alex
> >>>Yes but we'll still need to build two variants: fixed endian and
> >>>dynamic endian platforms.
> >>>Something along the lines of 32/64 bit split that we have?
> >>Why bother? Always make it dynamic and don't change the per-device
> >>variable, no? I'd be surprised if the performance difference is
> >>measurable. The bulk of the data we transfer gets copied raw anyway.
> >>
> >>
> >>Alex
> >This will have to be measured and proved by whoever's proposing the
> >patch, not by reviewers.  Platforms such as AMD which don't do
> >prediction well would be especially interesting to test on.
> 
> Sure, Greg, can you do that? I'm sure Michael has test cases
> available he can give you to measure performance on this.
> 
> Speaking of which, how does all of this work with vhost?
> 
> 
> Alex

I think that's missing.
As a first step, we need to disable vhost when
host/guest endian-ness do not match.

We could also add cross-endian support to vhost.

Or just wait a couple more months for virtio 1.0 which is fixed
endian-ness.


-- 
MST