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Date: Thu, 27 Aug 2015 13:53:45 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1/2] memory: allow zero size for
 adjust_endianness()
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>, Jason Wang <jasowang@redhat.com>, QEMU Developers <qemu-devel@nongnu.org>, Greg Kurz <gkurz@linux.vnet.ibm.com>

On Thu, Aug 27, 2015 at 11:49:32AM +0100, Peter Maydell wrote:
> 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

This just mirrors an API we have in kvm: if you pass 0
size when registering an ioeventfd, it will match on access
of any size.

-- 
MST