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

[Alexander Graf <agraf@suse.de>]

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?

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 :).


Alex