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

[Michael S. Tsirkin]

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.

> 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?

-- 
MST