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From: Peter Maydell <peter.maydell@linaro.org>
Date: Thu, 27 Aug 2015 11:49:32 +0100
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Subject: Re: [Qemu-devel] [PATCH 1/2] memory: allow zero size for
	adjust_endianness()
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To: Jason Wang <jasowang@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>, QEMU Developers <qemu-devel@nongnu.org>, Greg Kurz <gkurz@linux.vnet.ibm.com>

On 27 August 2015 at 05:50, Jason Wang <jasowang@redhat.com> wrote:
> On 08/26/2015 10:51 PM, Greg Kurz wrote:
>> On Wed, 26 Aug 2015 15:21:59 +0100
>> Peter Maydell <peter.maydell@linaro.org> wrote:
>>> This seems to me like a bug in the caller. Why would anything
>>> try to call into the memory subsystem to do a zero-size
>>> transaction?

>> Here's the patch which needs zero-size eventfd:
>>
>> http://patchwork.ozlabs.org/patch/509428/

> Yes, this is because we want to use wildcard mmio eventfd (which
> requires size to be zero) to speed up virtio 1.0 mmio.

But *why* does it require the size to be zero? I still think
the caller should just avoid trying to do zero-size memory
operations: they don't make sense. What is a zero size
operation supposed to mean?

-- PMM