Re: [Xen-ia64-devel] [PATCH] Fix unaligned reference of QEMU

[Alex Williamson <alex.williamson@xxxxxx>]

On Tue, 2007-08-21 at 14:58 +0800, Duan, Ronghui wrote:
> Hi
> 
> In current changeset.,qemu copy data uses memcpy_words, which copies 4
> bytes at once if the length is larger than 4. This causes unaligned
> reference and leads to a performance downgrade. The issue met in
> rtl8139 emulator. This patch fixes it.

Hi,

   I'd like to see this get fixed, but I'm not sure this is the right
way to do it.  Originally, there was a simple memcpy() here.   The
trouble started with this patch:

http://xenbits.xensource.com/xen-unstable.hg?rev/677731eb734d

This change required 8-byte alignment on 64 bit systems instead of the
4-byte claimed in the changelog, which resulted in this patch:

http://xenbits.xensource.com/xen-unstable.hg?rev/896b536d66c9

Now the problem appears to be that memcpy_words() is not isolated to
only the USB traffic.  When I see the unaligned accesses, they seem to
have some correlation with network traffic, and your benchmark results
prove that.  I think we need figure out when to use memcpy_words to
avoid the USB issue and when to use memcpy.  Maybe we can do that by
looking at the alignment and using the best one, or maybe it's possible
to distinguish data streams in exec-dm.  Thanks,

        Alex 

-- 
Alex Williamson                             HP Open Source & Linux Org.


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