Re: [Xen-devel] a ton of kernel issues

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2011-12-14 at 07:25 +0000, Ian Campbell wrote:
> 
> It controls precisely the behaviour you need! Try "maxmem=2048" and
> "memory=1024" in your guest configuration, it should boot with 1G of
> RAM and allow you to balloon to 2G and back. 

I take it back, there is indeed a bug in the PV ops kernel in this
regard.

It works with xm/xend because they set the maximum reservation for
guests to static-max on boot. xl (and, I think, xapi) instead set the
maximum reservation to the current balloon target and change it
dynamically as the target is changed (as a method of enforcing the
targets). However the pvops kernel incorrectly uses the maximum
reservation at boot to size the physical address space for guests.

The patch below fixes this.

Ian.

8<-------------------------------------------------------------

>From 649ca3b7ddca1cdda85c27e34f806f30484172ec Mon Sep 17 00:00:00 2001
From: Ian Campbell <ian.campbell@xxxxxxxxxx>
Date: Wed, 14 Dec 2011 12:00:38 +0000
Subject: [PATCH] xen: only limit memory map to maximum reservation for domain 0.

d312ae878b6a "xen: use maximum reservation to limit amount of usable RAM"
clamped the total amount of RAM to the current maximum reservation. This is
correct for dom0 but is not correct for guest domains. In order to boot a guest
"pre-ballooned" (e.g. with memory=1G but maxmem=2G) in order to allow for
future memory expansion the guest must derive max_pfn from the e820 provided by
the toolstack and not the current maximum reservation (which can reflect only
the current maximum, not the guest lifetime max). The existing algorithm
already behaves this correctly if we do not artificially limit the maximum
number of pages for the guest case.

For a guest booted with maxmem=512, memory=128 this results in:
 [    0.000000] BIOS-provided physical RAM map:
 [    0.000000]  Xen: 0000000000000000 - 00000000000a0000 (usable)
 [    0.000000]  Xen: 00000000000a0000 - 0000000000100000 (reserved)
-[    0.000000]  Xen: 0000000000100000 - 0000000008100000 (usable)
-[    0.000000]  Xen: 0000000008100000 - 0000000020800000 (unusable)
+[    0.000000]  Xen: 0000000000100000 - 0000000020800000 (usable)
...
 [    0.000000] NX (Execute Disable) protection: active
 [    0.000000] DMI not present or invalid.
 [    0.000000] e820 update range: 0000000000000000 - 0000000000010000 (usable) 
==> (reserved)
 [    0.000000] e820 remove range: 00000000000a0000 - 0000000000100000 (usable)
-[    0.000000] last_pfn = 0x8100 max_arch_pfn = 0x1000000
+[    0.000000] last_pfn = 0x20800 max_arch_pfn = 0x1000000
 [    0.000000] initial memory mapped : 0 - 027ff000
 [    0.000000] Base memory trampoline at [c009f000] 9f000 size 4096
-[    0.000000] init_memory_mapping: 0000000000000000-0000000008100000
-[    0.000000]  0000000000 - 0008100000 page 4k
-[    0.000000] kernel direct mapping tables up to 8100000 @ 27bb000-27ff000
+[    0.000000] init_memory_mapping: 0000000000000000-0000000020800000
+[    0.000000]  0000000000 - 0020800000 page 4k
+[    0.000000] kernel direct mapping tables up to 20800000 @ 26f8000-27ff000
 [    0.000000] xen: setting RW the range 27e8000 - 27ff000
 [    0.000000] 0MB HIGHMEM available.
-[    0.000000] 129MB LOWMEM available.
-[    0.000000]   mapped low ram: 0 - 08100000
-[    0.000000]   low ram: 0 - 08100000
+[    0.000000] 520MB LOWMEM available.
+[    0.000000]   mapped low ram: 0 - 20800000
+[    0.000000]   low ram: 0 - 20800000

With this change "xl mem-set <domain> 512M" will successfully increase the
guest RAM (by reducing the balloon).

There is no change for dom0.

Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
Cc: stable@xxxxxxxxxx
Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
 arch/x86/xen/setup.c |   18 +++++++++++++++---
 1 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index 38d0af4..a54ff1a 100644
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -173,9 +173,21 @@ static unsigned long __init xen_get_max_pages(void)
        domid_t domid = DOMID_SELF;
        int ret;
 
-       ret = HYPERVISOR_memory_op(XENMEM_maximum_reservation, &domid);
-       if (ret > 0)
-               max_pages = ret;
+       /*
+        * For the initial domain we use the maximum reservation as
+        * the maximum page.
+        *
+        * For guest domains the current maximum reservation reflects
+        * the current maximum rather than the static maximum. In this
+        * case the e820 map provided to us will cover the static
+        * maximum region.
+        */
+       if (xen_initial_domain()) {
+               ret = HYPERVISOR_memory_op(XENMEM_maximum_reservation, &domid);
+               if (ret > 0)
+                       max_pages = ret;
+       }
+
        return min(max_pages, MAX_DOMAIN_PAGES);
 }
 
-- 
1.7.2.5




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