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


From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH 2/2] memory: fix the eventfd data endianness according to the host
Date: Fri, 13 Mar 2015 15:52:50 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0


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

> 
>> 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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