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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] Re: [PATCH 2 of 5] add can_dma/post_dma for direct IO
Date: Mon, 15 Dec 2008 00:57:08 +0000
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To: qemu-devel@nongnu.org
Cc: Andrea Arcangeli <aarcange@redhat.com>, chrisw@redhat.com, kvm@vger.kernel.org, Gerd Hoffmann <kraxel@redhat.com>, Avi Kivity <avi@redhat.com>

> > That's pointless; cirrus for example has 8MB of mmio while a
> > cpu-to-vram blit is in progress, and some random device we'll add
> > tomorrow could easily introduce more.  Our APIs shouldn't depend on
> > properties of emulated hardware, at least as much as possible.
>
> One way to think of what I'm suggesting, is that if for every
> cpu_register_physical_memory call for MMIO, we allocated a buffer, then
> whenever map() was called on MMIO, we would return that already
> allocated buffer.  The overhead is fixed and honestly relatively small.
> Much smaller than dma.c proposes.

I Wouldn't be surprised if some machines had a large memory mapped IO space.  
Most of it might not be actively used, but once you start considering 64-bit 
machines on 32-bit hosts these allocations would become problematic.