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[Qemu-devel] Re: [PATCH] RFC: use logging count for individual regions


From: Avi Kivity
Subject: [Qemu-devel] Re: [PATCH] RFC: use logging count for individual regions
Date: Tue, 28 Jul 2009 09:36:42 +0300
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On 07/28/2009 04:08 AM, Glauber Costa wrote:
On Sat, Jul 25, 2009 at 11:28:20AM +0200, Jan Kiszka wrote:
Glauber Costa wrote:
qemu-kvm use this scheme of logging count of individual regions,
which is, IMHO, more flexible which the one we have right now.
I'm proposing we use it.

Anthony, please don't apply this patch yet, as I would want it
to receive proper testing, and FYI, current migration broken ;(
- and I don't really have time to go debug it now.

Jan: Please let me know what you think of it.
No principle concerns. But before looking into details: what additional
use cases will it cover (maybe some example from qemu-kvm), or what
existing code can it help to simplify?

Maybe avi can provide more input here, but to the very least, I believe this
approach is more proven, since it lived in qemu-kvm for a while now. Although 
more
cumbersome, the bits in avi's tree usually work better for kvm-related stuff.

I don't see a particular code path it simplifies, but I believe it can help us 
finding
bugs that will manifest in the form of an unbalanced count. It will also work 
if we ever
happen to have two entities manipulating dirty bits in the VGA region, like if 
we some day
implement dual head or something (although one might arguee that we should 
change it when
the time comes...)

Unlike the qemu dirty byte-map, the kvm dirty log can only service one user. As we have multiple users (vga and migration), we need some way to fix this impedance mismatch. I think refcounts are the easiest.

Btw, a side note: in your current scheme, what we do when migration fail? Do we 
keep migration_log
up ? I can't find any place in the code where we put it down

What about qemu-kvm?  Do we clear logging there?

--
Do not meddle in the internals of kernels, for they are subtle and quick to 
panic.





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