From: Jarod Wilson <jwilson@redhat.com>
Date: Mon, 18 Feb 2008 17:20:19 -0500
Subject: [xen] ia64: fix kernel panic on systems w/<=4GB RAM
Message-id: 200802181720.19405.jwilson@redhat.com
O-Subject: [RHEL5.2 PATCH] ia64 xen: fix kernel panic on systems w/<=4GB RAM
Bugzilla: 431001

Bug #431001: [RHEL5 U2] Kernel-xen Panic on CPU 1: ia64_fault, When trying to
install a xen guest
https://bugzilla.redhat.com/show_bug.cgi?id=431001

Description
-----------
A combination of xen changes have increased the amount of memory that the xen
hypervisor needs to save for itself to do its thing. Unfortunately, xen isn't
particularly bright about knowing how much memory it will need, particularly on
ia64. A system with only 2GB in rhts started hitting a kernel panic with recent
kernels when installing a xen guest, which went away when dom0_mem=1G was
specified. The basic problem is that the hypervisor didn't save enough memory
for its own use, it allocated too much to dom0. The dom0 memory allocation
routines basically have a fudge-factor for memory to reserve for the
hypervisor, and the simple fix is to increase it a bit. Saving an extra 64MB
prevented the panic on the machine in question, but still gave a bunch of 'bad
mpa' messages when attempting to install a guest. Saving an extra 128MB results
in no panic and no 'bad mpa' spew. Ugly, but so is ia64 xen in general... :)

Upstream Status
---------------
Not there yet, will take it to the upstream ia64 xen list soon.

This bug is marked as a Regression from 5.1 and it fixes a reproduceable kernel
panic. Please ACK.

Acked-by: Bill Burns <bburns@redhat.com>

diff --git a/arch/ia64/xen/domain.c b/arch/ia64/xen/domain.c
index f01b027..1ac0b94 100644
--- a/arch/ia64/xen/domain.c
+++ b/arch/ia64/xen/domain.c
@@ -1083,10 +1083,10 @@ static void calc_dom0_size(void)
 	 * for DMA and PCI mapping from the available domheap pages. The
 	 * chunk for DMA, PCI, etc., is a guestimate, as xen doesn't seem
 	 * to have a good idea of what those requirements might be ahead
-	 * of time, calculated at 1MB per 4GB of system memory */
+	 * of time, calculated at 128MB + 1MB per 4GB of system memory */
 	domheap_pages = avail_domheap_pages();
 	p2m_pages = domheap_pages / PTRS_PER_PTE;
-	spare_hv_pages = domheap_pages / 4096;
+	spare_hv_pages = 8192 + (domheap_pages / 4096);
 	max_dom0_size = (domheap_pages - (p2m_pages + spare_hv_pages))
 			 * PAGE_SIZE;
 	printk("Maximum permitted dom0 size: %luMB\n",