Re: [PATCH 13/13] xen: allow more than 512 GB of RAM for 64 bit pv-domains

From: Juergen Gross
Date: Wed Feb 18 2015 - 04:37:58 EST

On 02/18/2015 10:21 AM, Paul Bolle wrote:
On Wed, 2015-02-18 at 07:52 +0100, Juergen Gross wrote:
64 bit pv-domains under Xen are limited to 512 GB of RAM today. The
main reason has been the 3 level p2m tree, which was replaced by the
virtual mapped linear p2m list. Parallel to the p2m list which is
being used by the kernel itself there is a 3 level mfn tree for usage
by the Xen tools and eventually for crash dump analysis. For this tree
the linear p2m list can serve as a replacement, too. As the kernel
can't know whether the tools are capable of dealing with the p2m list
instead of the mfn tree, the limit of 512 GB can't be dropped in all

This patch replaces the hard limit by a kernel parameter which tells
the kernel to obey the 512 GB limit or not. The default is selected by
a configuration parameter which specifies whether the 512 GB limit
should be active per default for dom0 (only crash dump analysis is
affected) and/or for domUs (additionally domain save/restore/migration
are affected).

Memory above the domain limit is returned to the hypervisor instead of
being identity mapped, which was wrong anyways.

The kernel configuration parameter to specify the maximum size of a
domain can be deleted, as it is not relevant any more.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Documentation/kernel-parameters.txt | 7 ++++
arch/x86/include/asm/xen/page.h | 4 ---
arch/x86/xen/Kconfig | 31 +++++++++++-----
arch/x86/xen/p2m.c | 10 +++---
arch/x86/xen/setup.c | 72 ++++++++++++++++++++++++++++++-------
5 files changed, 93 insertions(+), 31 deletions(-)


--- a/arch/x86/xen/Kconfig
+++ b/arch/x86/xen/Kconfig
@@ -23,14 +23,29 @@ config XEN_PVHVM
def_bool y
depends on XEN && PCI && X86_LOCAL_APIC

- int
- default 500 if X86_64
- default 64 if X86_32
- depends on XEN
- help
- This only affects the sizing of some bss arrays, the unused
- portions of which are freed.
+if X86_64

&& XEN

The complete directory is made only if CONFIG_XEN is set.

+ prompt "Support pv-domains larger than 512GB"
+ default XEN_512GB_NONE
+ help
+ Support paravirtualized domains with more than 512GB of RAM.
+ The Xen tools and crash dump analysis tools might not support
+ pv-domains with more than 512 GB of RAM. This option controls the
+ default setting of the kernel to use only up to 512 GB or more.
+ It is always possible to change the default via specifying the
+ boot parameter "xen_512gb_limit".
+ config XEN_512GB_NONE
+ bool "neither dom0 nor domUs can be larger than 512GB"
+ config XEN_512GB_DOM0
+ bool "dom0 can be larger than 512GB, domUs not"
+ config XEN_512GB_DOMU
+ bool "domUs can be larger than 512GB, dom0 not"
+ config XEN_512GB_ALL
+ bool "dom0 and domUs can be larger than 512GB"

So there are actually two independent limits, configured through a
choice with four entries. Would using just two separate Kconfig symbols
(XEN_512GB_DOM0 and XEN_512GB_DOMU) without a choice wrapper also work?


Because ...




diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index 84a6473..16d94de 100644
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -32,6 +32,8 @@
#include "p2m.h"
#include "mmu.h"

+#define GB(x) ((uint64_t)(x) * 1024 * 1024 * 1024)
/* Amount of extra memory space we add to the e820 ranges */
struct xen_memory_region xen_extra_mem[XEN_EXTRA_MEM_MAX_REGIONS] __initdata;

@@ -85,6 +87,27 @@ static struct {
#define EXTRA_MEM_RATIO (10)

+static bool xen_dom0_512gb_limit __initdata =

... then this could be something like:
static bool xen_dom0_512gb_limit __initdata = !IS_ENABLED(CONFIG_XEN_512GB_DOM0);

+static bool xen_domu_512gb_limit __initdata =

and this likewise:
static bool xen_domu_512gb_limit __initdata = !IS_ENABLED(CONFIG_XEN_512GB_DOMU);



That's a matter of taste, I think.

+static int __init xen_parse_512gb(char *arg)
+ bool val = false;
+ if (!arg)
+ val = true;
+ else if (strtobool(arg, &val))
+ return 1;
+ xen_dom0_512gb_limit = val;
+ xen_domu_512gb_limit = val;
+ return 0;
+early_param("xen_512gb_limit", xen_parse_512gb);
static void __init xen_add_extra_mem(phys_addr_t start, phys_addr_t size)
int i;

So one can configure these two limits separately, but the kernel
parameter is used for both. Any particular reason?

Yes. A kernel is running only either as Dom0 or as domU at a given time.
Having two parameters here would be nonsense, as only one could apply.

And being able to configure both limits separately does make sense,
of course.

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