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Re: Sudden slowdown of ARM emulation in master

From: Philippe Mathieu-Daudé
Subject: Re: Sudden slowdown of ARM emulation in master
Date: Wed, 26 Feb 2020 09:51:01 +0100
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On 2/26/20 9:48 AM, Peter Maydell wrote:
On Wed, 26 Feb 2020 at 08:44, Philippe Mathieu-Daudé <address@hidden> wrote:

On 2/26/20 9:41 AM, Peter Maydell wrote:
On Tue, 25 Feb 2020 at 23:08, Niek Linnenbank <address@hidden> wrote:

Just now I was working on some small fixes for the cubieboard machine and 
rebasing my Allwinner H3 branches.
While doing some testing, I noticed that suddenly the machines were much slower 
than before.
I only see this happening when I rebase to this commit:
     ca6155c0f2bd39b4b4162533be401c98bd960820 ("Merge tag 'patchew/address@hidden' 
of https://github.com/patchew-project/qemu into HEAD")

Yeah, I noticed a slowdown yesterday as well, but haven't tracked it down
as yet. The first thing would be to do a git bisect to try to narrow
down what commit caused it.

My guess: biggest chunk of memory is the DRAM, registered as "fast RAM"
by QEMU, but the SoCs provide SRAM which is supposed to be faster. Not
anymore with QEMU. And Linux try to use the SRAM when possible.

Doesn't sound very likely to me: generally Linux doesn't use random small
lumps of SRAM, it just goes for whatever the dtb says is the main RAM,
usually DRAM. And I thought that all RAM blocks within QEMU performed
the same?

 From the commit that Howard tracked down as the cause it looks like
an ordering-of-actions issue in vl.c where something that was assuming
memory-size-related stuff was set up is now running before those
variables/fields are set correctly rather than after ?

Yes, I just saw Howard's email and was going to reply to my own "I'm probably wrong since the mac99 machine doesn't use SRAM".

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