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

[Greg Kurz]

On Mon, 14 Apr 2014 15:08:23 +0200
Alexander Graf <address@hidden> wrote:

> 
> On 14.04.14 14:55, Michael S. Tsirkin wrote:
> > 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.
> > Measuring performance is hard ATM.
> >
> > Disk io stress when backed by raw ramdisk in host is usually the easiest
> > way to stress userspace virtio.
> > You need to make sure your baseline is repeateable though:
> > pin down everything from cpu to hardware interrupts,
> > disable power management, c states etc
> > Just taking an average and hoping for the best doesn't cut it.
> >
> >
> > We really should write a benchmark device,
> > to measure just the transport overhead.
> > For example, start with virtio-net but fuse TX and RX
> > VQs together, so you'll get right back whatever you send out.
> >
> > So really, virtio is ATM target-specific and I think it's
> > a good idea to get it working as such first,
> > do extra cleanups like getting rid of target specific code
> > separately.
> 
> How does it help when we keep it target specific? The only way to remove 
> any overhead from this refactoring would be to instead of
> 
>    if (vdev->is_big_endian)
> 
> write it as
> 
>    if (virtio_is_big_endian_device(vdev))
> 
> which we could define as
> 
> static inline bool virtio_is_big_endian_device(VirtIODevice *vdev)
> {
> #if defined(TARGET_PPC)
>    return vdev->is_big_endian
> #elif defined(TARGET_WORDS_BIGENDIAN)
>    return true;
> #else
>    return false;
> #endif
> }
> 
> If it makes you happy to do it this way first and move virtio to 
> target-independent files later, we can certainly do this.
> 
> 

This would end up defined in all virtio files with the consequence of
hitting TARGET_WORDS_BIGENDIAN poison for the target independent ones.
Should we kick them out of common-obj variables in makefiles as well ?
Or would it be acceptable to have this helper not inlined ?

> Alex
> 


Thanks.

--
Greg