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Date: Wed, 4 Aug 2010 18:01:54 +0300
From: Gleb Natapov <gleb@redhat.com>
Subject: Re: [Qemu-devel] Anyone seeing huge slowdown launching qemu with
	Linux 2.6.35?
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: qemu-devel@nongnu.org, Gerd Hoffmann <kraxel@redhat.com>, "Richard W.M. Jones" <rjones@redhat.com>, kvm@vger.kernel.org, Avi Kivity <avi@redhat.com>

On Wed, Aug 04, 2010 at 09:50:55AM -0500, Anthony Liguori wrote:
> On 08/04/2010 09:38 AM, Gleb Natapov wrote:
> >>
> >>But even if it wasn't it can potentially create havoc.  I think we
> >>currently believe that the northbridge likely never forwards RAM
> >>access to a device so this doesn't fit how hardware would work.
> >>
> >Good point.
> >
> >>More importantly, BIOSes and ROMs do very funny things with RAM.
> >>It's not unusual for a ROM to muck with the e820 map to allocate RAM
> >>for itself which means there's always the chance that we're going to
> >>walk over RAM being used for something else.
> >>
> >ROM does not muck with the e820. It uses PMM to allocate memory and the
> >memory it gets is marked as reserved in e820 map.
> 
> PMM allocations are only valid during the init function's execution.
> It's intention is to enable the use of scratch memory to decompress
> or otherwise modify the ROM to shrink its size.
> 
Hm, may be. I read seabios code differently, but may be I misread it.
 
> If a ROM needs memory after the init function, it needs to use the
> traditional tricks to allocate long term memory and the most popular
> one is modifying the e820 tables.
> 
e820 has no in memory format,

> See src/arch/i386/firmware/pcbios/e820mangler.S in gPXE.
so this ugly code intercepts int15 and mangle result. OMG. How this can
even work if more then two ROMs want to do that?

--
			Gleb.