Re: [Qemu-devel] [PATCH 2/2] memory: fix the eventfd data endianness according to the host

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

On Fri, 13 Mar 2015 12:06:06 +0100
Paolo Bonzini <pbonzini@redhat.com> wrote:
> 
> 
> On 13/03/2015 09:11, Greg Kurz wrote:
> > The data argument is a host entity. It is not related to the target
> > endianness. Let's introduce a HOST_WORDS_BIGENDIAN based helper for
> > that.
> > 
> > This patch fixes ioeventfd and vhost for a ppc64le host running a ppc64le
> > guest (only virtqueue 0 was handled, all others being byteswapped because
> > of TARGET_WORDS_BIGENDIAN). It doesn't change functionnality for fixed
> > endian architectures (i.e. doesn't break x86).
> > 
> > Reported-by: Cédric Le Goater <clg@fr.ibm.com>
> > Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > ---
> >  memory.c |   13 +++++++++++--
> >  1 file changed, 11 insertions(+), 2 deletions(-)
> > 
> > diff --git a/memory.c b/memory.c
> > index 6291cc0..1e29d40 100644
> > --- a/memory.c
> > +++ b/memory.c
> > @@ -1549,6 +1549,15 @@ void memory_region_clear_flush_coalesced(MemoryRegion *mr)
> >      }
> >  }
> >  
> > +static bool eventfd_wrong_endianness(MemoryRegion *mr)
> > +{
> > +#ifdef HOST_WORDS_BIGENDIAN
> > +    return mr->ops->endianness == DEVICE_LITTLE_ENDIAN;
> > +#else
> > +    return mr->ops->endianness == DEVICE_BIG_ENDIAN;
> > +#endif
> > +}
> > +
> >  void memory_region_add_eventfd(MemoryRegion *mr,
> >                                 hwaddr addr,
> >                                 unsigned size,
> > @@ -1565,7 +1574,7 @@ void memory_region_add_eventfd(MemoryRegion *mr,
> >      };
> >      unsigned i;
> >  
> > -    adjust_endianness(&mrfd.data, size, memory_region_wrong_endianness(mr));
> > +    adjust_endianness(&mrfd.data, size, eventfd_wrong_endianness(mr));
> 
> Strictly speaking, the place to do this would be kvm_set_ioeventfd_mmio.

FWIW the swap is being done in memory.c since commit:

commit 28f362be6e7f45ea9b7a57a08555c4c784f36198
Author: Alexander Graf <agraf@suse.de>
Date:   Mon Oct 15 20:30:28 2012 +0200

    memory: Make eventfd adhere to device endianness

Are you asking to revert this commit and to pass the device endianness to
kvm_set_ioeventfd_mmio() so it can fix the ordering ?

>  A hypothetical userspace ioeventfd emulation would not need the swap.
> 

I don't understand why "would not need the swap"...


> I can accept the patch, but it's better to add a comment.
> 

... and so I don't know what to write. :)

Please enlight !

--
Greg

> Paolo
> 
> >      memory_region_transaction_begin();
> >      for (i = 0; i < mr->ioeventfd_nb; ++i) {
> >          if (memory_region_ioeventfd_before(mrfd, mr->ioeventfds[i])) {
> > @@ -1598,7 +1607,7 @@ void memory_region_del_eventfd(MemoryRegion *mr,
> >      };
> >      unsigned i;
> >  
> > -    adjust_endianness(&mrfd.data, size, memory_region_wrong_endianness(mr));
> > +    adjust_endianness(&mrfd.data, size, eventfd_wrong_endianness(mr));
> >      memory_region_transaction_begin();
> >      for (i = 0; i < mr->ioeventfd_nb; ++i) {
> >          if (memory_region_ioeventfd_equal(mrfd, mr->ioeventfds[i])) {
> > 
>