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[Xen-devel] [PATCH for-4.13 2/2] Rationalize max_grant_frames and max_maptrack_frames handling



Xen used to have single, system-wide limits for the number of grant
frames and maptrack frames a guest was allowed to create.  Increasing
or decreasing this single limit on the Xen command-line would change
the limit for all guests on the system.

Later, per-domain limits for these values was created.  The
system-wide limits became strict limits: domains could not be created
with higher limits, but could be created with lower limits.

However, the change also introduced a range of different "default"
values into various places in the toolstack:

- The python libxc bindings hard-coded these values to 32 and 1024,
  respectively

- The libxl default values are 32 and 1024 respectively.

- xl will use the libxl default for maptrack, but does its own default
  calculation for grant frames: either 32 or 64, based on the max
  possible mfn.

These defaults interact poorly with the hypervisor command-line limit:

- The hypervisor command-line limit cannot be used to raise the limit
  for all guests anymore, as the default in the toolstack will
  effectively override this.

- If you use the hypervisor command-line limit to *reduce* the limit,
  then the "default" values generated by the toolstack are too high,
  and all guest creations will fail.

In other words, the toolstack defaults require any change to be
effected by having the admin explicitly specify a new value in every
guest.

In order to address this, have grant_table_init treat '0' values for
max_grant_frames and max_maptrack_frames as instructions to use the
system-wide default.  Have all the above toolstacks default to passing
0 unless a different value is explicitly given.

This restores the old behavior, that changing the hypervisor
command-line option can change the behavior for all guests, while
retaining the ability to set per-guest values.  It also removes the
bug that *reducing* the system-wide max will cause all domains without
explicit limits to fail.

(The ocaml bindings require the caller to always specify a value, and
the code to start a xenstored stubdomain hard-codes these to 4 and 128
respectively; these will not be addressed here.)

Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
---
Release justification: This is an observed regression (albeit one that
has spanned several releases now).

Compile-tested only.

NB this patch could be applied without the whitespace fixes (perhaps
with some fix-ups); it's just easier since my editor strips trailing
whitespace out automatically.

CC: Ian Jackson <ian.jackson@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Jan Beulich <jbeulich@xxxxxxxx>
CC: Paul Durrant <paul@xxxxxxx>
CC: Julien Grall <julien@xxxxxxx>
CC: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Juergen Gross <jgross@xxxxxxxx>
CC: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
---
 tools/libxl/libxl.h               |  4 ++--
 tools/python/xen/lowlevel/xc/xc.c |  2 --
 tools/xl/xl.c                     | 12 ++----------
 xen/common/grant_table.c          |  7 +++++++
 xen/include/public/domctl.h       |  6 ++++--
 5 files changed, 15 insertions(+), 16 deletions(-)

diff --git a/tools/libxl/libxl.h b/tools/libxl/libxl.h
index 49b56fa1a3..1648d337e7 100644
--- a/tools/libxl/libxl.h
+++ b/tools/libxl/libxl.h
@@ -364,8 +364,8 @@
  */
 #define LIBXL_HAVE_BUILDINFO_GRANT_LIMITS 1
 
-#define LIBXL_MAX_GRANT_FRAMES_DEFAULT 32
-#define LIBXL_MAX_MAPTRACK_FRAMES_DEFAULT 1024
+#define LIBXL_MAX_GRANT_FRAMES_DEFAULT 0
+#define LIBXL_MAX_MAPTRACK_FRAMES_DEFAULT 0
 
 /*
  * LIBXL_HAVE_BUILDINFO_* indicates that libxl_domain_build_info has
diff --git a/tools/python/xen/lowlevel/xc/xc.c 
b/tools/python/xen/lowlevel/xc/xc.c
index 6d2afd5695..0f861872ce 100644
--- a/tools/python/xen/lowlevel/xc/xc.c
+++ b/tools/python/xen/lowlevel/xc/xc.c
@@ -127,8 +127,6 @@ static PyObject *pyxc_domain_create(XcObject *self,
         },
         .max_vcpus = 1,
         .max_evtchn_port = -1, /* No limit. */
-        .max_grant_frames = 32,
-        .max_maptrack_frames = 1024,
     };
 
     static char *kwd_list[] = { "domid", "ssidref", "handle", "flags",
diff --git a/tools/xl/xl.c b/tools/xl/xl.c
index ddd29b3f1b..b6e220184d 100644
--- a/tools/xl/xl.c
+++ b/tools/xl/xl.c
@@ -51,8 +51,8 @@ libxl_bitmap global_pv_affinity_mask;
 enum output_format default_output_format = OUTPUT_FORMAT_JSON;
 int claim_mode = 1;
 bool progress_use_cr = 0;
-int max_grant_frames = -1;
-int max_maptrack_frames = -1;
+int max_grant_frames = 0;
+int max_maptrack_frames = 0;
 
 xentoollog_level minmsglevel = minmsglevel_default;
 
@@ -96,7 +96,6 @@ static void parse_global_config(const char *configfile,
     XLU_Config *config;
     int e;
     const char *buf;
-    libxl_physinfo physinfo;
 
     config = xlu_cfg_init(stderr, configfile);
     if (!config) {
@@ -199,13 +198,6 @@ static void parse_global_config(const char *configfile,
 
     if (!xlu_cfg_get_long (config, "max_grant_frames", &l, 0))
         max_grant_frames = l;
-    else {
-        libxl_physinfo_init(&physinfo);
-        max_grant_frames = (libxl_get_physinfo(ctx, &physinfo) != 0 ||
-                            !(physinfo.max_possible_mfn >> 32))
-                           ? 32 : 64;
-        libxl_physinfo_dispose(&physinfo);
-    }
     if (!xlu_cfg_get_long (config, "max_maptrack_frames", &l, 0))
         max_maptrack_frames = l;
 
diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index b34d520f6d..cd24029e33 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -1843,6 +1843,13 @@ int grant_table_init(struct domain *d, unsigned int 
max_grant_frames,
     struct grant_table *gt;
     int ret = -ENOMEM;
 
+    /* Default to maximum values if no lower ones are specified */
+    if ( !max_grant_frames )
+        max_grant_frames = opt_max_grant_frames;
+
+    if ( !max_maptrack_frames )
+        max_maptrack_frames = opt_max_maptrack_frames;
+
     if ( max_grant_frames < INITIAL_NR_GRANT_FRAMES ||
          max_grant_frames > opt_max_grant_frames ||
          max_maptrack_frames > opt_max_maptrack_frames )
diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
index 9f2cfd602c..27d04f67aa 100644
--- a/xen/include/public/domctl.h
+++ b/xen/include/public/domctl.h
@@ -82,8 +82,10 @@ struct xen_domctl_createdomain {
     uint32_t iommu_opts;
 
     /*
-     * Various domain limits, which impact the quantity of resources (global
-     * mapping space, xenheap, etc) a guest may consume.
+     * Various domain limits, which impact the quantity of resources
+     * (global mapping space, xenheap, etc) a guest may consume.  For
+     * max_grant_frames and max_maptrack_frames, "0" means "use the
+     * default maximum value".
      */
     uint32_t max_vcpus;
     uint32_t max_evtchn_port;
-- 
2.24.0


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