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[Qemu-devel] Change of max-ram-below-4g initial value breaks Xen

From: Anthony PERARD
Subject: [Qemu-devel] Change of max-ram-below-4g initial value breaks Xen
Date: Wed, 22 Jun 2016 16:09:37 +0100
User-agent: Mutt/1.6.1 (2016-04-27)


following commit 8156d48 (pc: allow raising low memory via
max-ram-below-4g option), guest under Xen fail to boot. This is due to
the initial value of max-ram-below-4g been change from 4G to 3.5G.
Unfortunately, a Xen guest have a higher default split, at 3.75G, but
can be change via the -machine options max-ram-below-4g. Now, it looks
like a user ask for 3.5G where the rest of the Xen tool stack is still
at 3.75G.

In QEMU, the ram split calculation for Xen is done in xen_ram_init(). It
can be change via max-ram-below-4g.

The hvmloader firmware will start the MMIO hole at 3.75G unless told
otherwise via a xenstore entry. The xenstore entry can be set by the Xen
toolstack, which would also add 'max-ram-below-4g' to -machine if a user
request it.

How could xen_ram_init() find out if the value of max-ram-below-4g is
the default or if a user have set it? Is there another way we could fix


Anthony PERARD

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