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


From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH 2/2] memory: fix the eventfd data endianness according to the host
Date: Fri, 13 Mar 2015 16:24:03 +0100

On Fri, Mar 13, 2015 at 03:52:50PM +0100, Paolo Bonzini wrote:
> 
> 
> On 13/03/2015 12:32, Greg Kurz wrote:
> > On Fri, 13 Mar 2015 12:06:06 +0100
> > Paolo Bonzini <address@hidden> 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: C├ędric Le Goater <address@hidden>
> >>> Signed-off-by: Greg Kurz <address@hidden>
> >>> ---
> >>>  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 <address@hidden>
> > 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 ?
> 
> Yes.
> 
> >>  A hypothetical userspace ioeventfd emulation would not need the swap.
> > 
> > I don't understand why "would not need the swap"...
> 
> Because the userspace ioeventfd emulation would look at the value as it
> comes from the target CPU, in target CPU endianness.  So it would have a
> swap done at ioeventfd time, and a swap done at access time.  Host
> endianness doesn't matter.
> 
> Here, QEMU believes that the target's natural endianness is big-endian,
> but the kernel believes that the target's natural endianness is
> little-endian (based on MSR_KERNEL).  So there is a different number of
> swaps in memory_region_add_eventfd and in the kernel's kvmppc_handle_store.
> 
> Does something like this work?
> 
> #if defined(HOST_WORDS_BIGENDIAN) != defined(TARGET_WORDS_BIGENDIAN)
>     /* The kernel expects ioeventfd values in HOST_WORDS_BIGENDIAN
>      * endianness, but the memory core hands them in target endianness.
>      * For example, PPC is always treated as big-endian even if running
>      * on KVM and on PPC64LE.  Correct here.
>      */
>     switch(size) {
>     case 2:
>         val = bswap16(val);
>         break;
>     case 4:
>         val = bswap32(val);
>         break;
>     }
> #endif
> 
> in kvm_set_ioeventfd_mmio and kvm_set_ioeventfd_pio?
> 
> Paolo

Maybe you can post a patch? Would be easier ...

> > 
> >> 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])) {
> >>>
> >>
> > 



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