Re: [Qemu-devel] [PATCH v6 1/8] virtio_legacy_get_byteswap: endian-ambivalent targets using legacy virtio

[Greg Kurz <gkurz@linux.vnet.ibm.com>]

On Mon, 31 Mar 2014 16:50:55 +0200
Alexander Graf <agraf@suse.de> wrote:

> On 03/28/2014 11:57 AM, Greg Kurz wrote:
> > From: Rusty Russell <rusty@rustcorp.com.au>
> >
> > 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 all little endian.
> >
> > We need to support both implementations and we want to share as much code
> > as possible.
> >
> > A good way to do it is to introduce a per-device boolean property to tell
> > memory accessors whether they should swap bytes or not. This flag should
> > be set at device reset time, because:
> > - endianness won't change while the device is in use, and if we reboot
> >    into a different endianness, a new device reset will occur
> > - as suggested by Alexander Graf, we can keep all the logic to set the
> >    property in a single place and share all the virtio memory accessors
> >    between the two implementations
> >
> > For legacy devices, we rely on a per-platform hook to set the flag. The
> > virtio 1.0 implementation will just have to add some more logic in
> > virtio_reset() instead of calling the hook:
> >
> > if (vdev->legacy) {
> >     vdev->needs_byteswap = virtio_legacy_get_byteswap();
> > } else {
> > #ifdef HOST_WORDS_BIGENDIAN
> >     vdev->needs_byteswap = true;
> > #else
> >     vdev->needs_byteswap = false;
> > #endif
> > }
> >
> > The needs_byteswap flag is preserved accross migrations.
> >
> > Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> > [ fixed checkpatch.pl error with the virtio_byteswap initialisation,
> >    ldq_phys() API change,
> >    relicensed virtio-access.h to GPLv2+ on Rusty's request,
> >    introduce a per-device needs_byteswap flag,
> >    add VirtIODevice * arg to virtio helpers,
> >    rename virtio_get_byteswap to virtio_legacy_get_byteswap,
> >    Greg Kurz <gkurz@linux.vnet.ibm.com> ]
> > Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > ---
> >   hw/virtio/virtio.c                |    5 +
> >   include/hw/virtio/virtio-access.h |  139 +++++++++++++++++++++++++++++++++++++
> >   include/hw/virtio/virtio.h        |    3 +
> >   stubs/Makefile.objs               |    1
> >   stubs/virtio_get_byteswap.c       |    6 ++
> >   5 files changed, 154 insertions(+)
> >   create mode 100644 include/hw/virtio/virtio-access.h
> >   create mode 100644 stubs/virtio_get_byteswap.c
> >
> > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > index aeabf3a..24b565f 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->needs_byteswap = virtio_legacy_get_byteswap();
> > +
> >       if (k->reset) {
> >           k->reset(vdev);
> >       }
> > @@ -845,6 +848,7 @@ void virtio_save(VirtIODevice *vdev, QEMUFile *f)
> >   
> >       qemu_put_8s(f, &vdev->status);
> >       qemu_put_8s(f, &vdev->isr);
> > +    qemu_put_8s(f, (uint8_t *) &vdev->needs_byteswap);
> >       qemu_put_be16s(f, &vdev->queue_sel);
> >       qemu_put_be32s(f, &vdev->guest_features);
> >       qemu_put_be32(f, vdev->config_len);
> > @@ -905,6 +909,7 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f)
> >   
> >       qemu_get_8s(f, &vdev->status);
> >       qemu_get_8s(f, &vdev->isr);
> > +    qemu_get_8s(f, (uint8_t *) &vdev->needs_byteswap);
> >       qemu_get_be16s(f, &vdev->queue_sel);
> >       qemu_get_be32s(f, &features);
> >   
> > diff --git a/include/hw/virtio/virtio-access.h b/include/hw/virtio/virtio-access.h
> > new file mode 100644
> > index 0000000..70dd1e2
> > --- /dev/null
> > +++ b/include/hw/virtio/virtio-access.h
> > @@ -0,0 +1,139 @@
> > +/*
> > + * Virtio Accessor Support: In case your target can change endian.
> > + *
> > + * Copyright IBM, Corp. 2013
> > + *
> > + * Authors:
> > + *  Rusty Russell   <rusty@au.ibm.com>
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License as published by
> > + * the Free Software Foundation, either version 2 of the License, or
> > + * (at your option) any later version.
> > + *
> > + */
> > +#ifndef _QEMU_VIRTIO_ACCESS_H
> > +#define _QEMU_VIRTIO_ACCESS_H
> > +#include "hw/virtio/virtio.h"
> > +
> > +static inline uint16_t virtio_lduw_phys(AddressSpace *as, hwaddr pa,
> > +                                        struct VirtIODevice *vdev)
> > +{
> > +    if (vdev->needs_byteswap) {
> > +        return bswap16(lduw_phys(as, pa));
> > +    }
> > +    return lduw_phys(as, pa);
> > +}
> > +
> > +static inline uint32_t virtio_ldl_phys(AddressSpace *as, hwaddr pa,
> > +                                       struct VirtIODevice *vdev)
> > +{
> > +    if (vdev->needs_byteswap) {
> > +        return bswap32(ldl_phys(as, pa));
> > +    }
> > +    return ldl_phys(as, pa);
> > +}
> > +
> > +static inline uint64_t virtio_ldq_phys(AddressSpace *as, hwaddr pa,
> > +                                       struct VirtIODevice *vdev)
> > +{
> > +    if (vdev->needs_byteswap) {
> > +        return bswap64(ldq_phys(as, pa));
> > +    }
> > +    return ldq_phys(as, pa);
> > +}
> > +
> > +static inline void virtio_stw_phys(AddressSpace *as, hwaddr pa, uint16_t value,
> > +                                   struct VirtIODevice *vdev)
> > +{
> > +    if (vdev->needs_byteswap) {
> > +        stw_phys(as, pa, bswap16(value));
> > +    } else {
> > +        stw_phys(as, pa, value);
> > +    }
> > +}
> > +
> > +static inline void virtio_stl_phys(AddressSpace *as, hwaddr pa, uint32_t value,
> > +                                   struct VirtIODevice *vdev)
> > +{
> > +    if (vdev->needs_byteswap) {
> > +        stl_phys(as, pa, bswap32(value));
> > +    } else {
> > +        stl_phys(as, pa, value);
> > +    }
> > +}
> > +
> > +static inline void virtio_stw_p(void *ptr, uint16_t v,
> > +                                struct VirtIODevice *vdev)
> > +{
> > +    if (vdev->needs_byteswap) {
> > +        stw_p(ptr, bswap16(v));
> > +    } else {
> > +        stw_p(ptr, v);
> > +    }
> > +}
> > +
> > +static inline void virtio_stl_p(void *ptr, uint32_t v,
> > +                                struct VirtIODevice *vdev)
> > +{
> > +    if (vdev->needs_byteswap) {
> > +        stl_p(ptr, bswap32(v));
> > +    } else {
> > +        stl_p(ptr, v);
> > +    }
> > +}
> > +
> > +static inline void virtio_stq_p(void *ptr, uint64_t v,
> > +                                struct VirtIODevice *vdev)
> > +{
> > +    if (vdev->needs_byteswap) {
> > +        stq_p(ptr, bswap64(v));
> > +    } else {
> > +        stq_p(ptr, v);
> > +    }
> > +}
> > +
> > +static inline int virtio_lduw_p(const void *ptr, struct VirtIODevice *vdev)
> > +{
> > +    if (vdev->needs_byteswap) {
> > +        return bswap16(lduw_p(ptr));
> > +    } else {
> > +        return lduw_p(ptr);
> > +    }
> > +}
> > +
> > +static inline int virtio_ldl_p(const void *ptr, struct VirtIODevice *vdev)
> > +{
> > +    if (vdev->needs_byteswap) {
> > +        return bswap32(ldl_p(ptr));
> > +    } else {
> > +        return ldl_p(ptr);
> > +    }
> > +}
> > +
> > +static inline uint64_t virtio_ldq_p(const void *ptr, struct VirtIODevice *vdev)
> > +{
> > +    if (vdev->needs_byteswap) {
> > +        return bswap64(ldq_p(ptr));
> > +    } else {
> > +        return ldq_p(ptr);
> > +    }
> > +}
> > +
> > +static inline uint32_t virtio_tswap32(uint32_t s, struct VirtIODevice *vdev)
> > +{
> > +    if (vdev->needs_byteswap) {
> > +        return bswap32(tswap32(s));
> > +    } else {
> > +        return tswap32(s);
> > +    }
> > +}
> > +
> > +static inline void virtio_tswap32s(uint32_t *s, struct VirtIODevice *vdev)
> > +{
> > +    tswap32s(s);
> > +    if (vdev->needs_byteswap) {
> > +        *s = bswap32(*s);
> > +    }
> 
> Couldn't you just reuse virtio_tswap32() here?
> 

Yes. Good catch !

> Also, I think a "needs_byteswap" flag gets confusing. Devices shouldn't 
> have any access to CPU endian specific RAM accessors.
> 
> How about we introduce a flag like "is_bigendian" that we set depending 
> on the CPU default endianness for legacy virtio. We can then change that 
> flag accordingly. For the wrappers here, we could just use accessors 
> like ldl_be_phys() or ldl_le_phys().
> 
> That should make things a lot easier to understand - and for virtio 1.0 
> we only need to set is_bigendian = false :).
> 

Yeah definitely !

> 
> Alex
> 
> 

Thanks.

-- 
Gregory Kurz                                     kurzgreg@fr.ibm.com
                                                 gkurz@linux.vnet.ibm.com
Software Engineer @ IBM/Meiosys                  http://www.ibm.com
Tel +33 (0)562 165 496

"Anarchy is about taking complete responsibility for yourself."
        Alan Moore.