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From: Marcel Apfelbaum <marcel.a@redhat.com>
Date: Fri, 08 Nov 2013 18:19:46 +0200
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Subject: Re: [Qemu-devel] [PATCH] exec: fix regression by making
 system-memory region UINT64_MAX size
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, "Michael S. Tsirkin" <mst@redhat.com>, Jan Kiszka <jan.kiszka@siemens.com>, "edk2-devel@lists.sourceforge.net" <edk2-devel@lists.sourceforge.net>, qemu-devel <qemu-devel@nongnu.org>, Laszlo Ersek <lersek@redhat.com>, Anthony Liguori <anthony@codemonkey.ws>, Jordan Justen <jljusten@gmail.com>, afaerber <afaerber@suse.de>, Richard Henderson <rth@twiddle.net>

On Fri, 2013-11-08 at 17:12 +0100, Paolo Bonzini wrote:
> Il 08/11/2013 16:08, Marcel Apfelbaum ha scritto:
> > Actually, as I see, the default behavior of "system" memory region
> > is to use unassigned_mem_ops that has valid.accepts method returning
> > false (no read/write methods). I don't see that read all-ones/ignore
> > writes is implemented.
> 
> Right, it's read-all-zeroes (see unassigned_mem_read).  It was
> read-all-ones for a brief time, then it got changed back to read-all-zeroes.
I remember something, Jan's patches got reverted because the modification
was system wide and not only for pci address space.
> 
> > This was the main reason I submitted this patch. I had *no* clue that
> > it would impact so much the system...
> 
> Yeah, it's not your fault.  But it should have been held back until 1.8.
>  Do you agree with reverting it?
Given the actual impact on version's stability, I agree
it is the right thing to do.

> 
> > I still think the patch is needed ans the guests will benefit from
> > more accurate PCI spec emulation.
> 
> Only buggy guests :) but yes, I agree it's a good thing to have.
Yes, it may be a driver there making some a wrong decision and
"reading-all-ones" may give it a clue. (the same for writes)

Thanks,
Marcel

> 
> Paolo