libxl, docs: Use arch-specific default paging memory

The default paging memory (descibed in `shadow_memory` entry in xl
config) in libxl is used to determine the memory pool size for xl
guests. Currently this size is only used for x86, and contains a part
of RAM to shadow the resident processes. Since on Arm there is no
shadow mode guests, so the part of RAM to shadow the resident processes
is not necessary. Therefore, this commit splits the function
`libxl_get_required_shadow_memory()` to arch specific helpers and
renamed the helper to `libxl__arch_get_required_paging_memory()`.

On x86, this helper calls the original value from
`libxl_get_required_shadow_memory()` so no functional change intended.

On Arm, this helper returns 1MB per vcpu plus 4KB per MiB of RAM
for the P2M map and additional 512KB.

Also update the xl.cfg documentation to add Arm documentation
according to code changes and correct the comment style following Xen
coding style.

This is part of CVE-2022-33747 / XSA-409.

Suggested-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Henry Wang <Henry.Wang@arm.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
master commit: 156a239ea288972425f967ac807b3cb5b5e14874
master date: 2022-10-11 14:28:37 +0200

diff --git a/docs/man/xl.cfg.5.pod.in b/docs/man/xl.cfg.5.pod.in
index 245d3f94728a3d3574830f0855f9ff61b37390ca..3b297c6a97f0407fee73ffe59a16f0b2502889d0 100644 (file)
--- a/docs/man/xl.cfg.5.pod.in
+++ b/docs/man/xl.cfg.5.pod.in
@@ -1790,6 +1790,11 @@ are not using hardware assisted paging (i.e. you are using shadow
 mode) and your guest workload consists of a very large number of
 similar processes then increasing this value may improve performance.
 
+On Arm, this field is used to determine the size of the guest P2M pages
+pool, and the default value is 1MB per vCPU plus 4KB per MB of RAM for
+the P2M map. Users should adjust this value if bigger P2M pool size is
+needed.
+
 =back
 
 =head3 Processor and Platform Features

diff --git a/tools/libxl/libxl_arch.h b/tools/libxl/libxl_arch.h
index 6a91775b9e20699b27b0262790276635db3b5724..b09f868490acffab78a8c85d34e2f98ff7476fa9 100644 (file)
--- a/tools/libxl/libxl_arch.h
+++ b/tools/libxl/libxl_arch.h
@@ -83,6 +83,10 @@ int libxl__arch_extra_memory(libxl__gc *gc,
                              const libxl_domain_build_info *info,
                              uint64_t *out);
 
+_hidden
+unsigned long libxl__arch_get_required_paging_memory(unsigned long maxmem_kb,
+                                                     unsigned int smp_cpus);
+
 #if defined(__i386__) || defined(__x86_64__)
 
 #define LAPIC_BASE_ADDRESS  0xfee00000

diff --git a/tools/libxl/libxl_arm.c b/tools/libxl/libxl_arm.c
index 34f8a29056db48b57e4fe3582ce72589f0def5fd..f4b3dc8e71396337cfbdcb4cba8a7e22b7a08d95 100644 (file)
--- a/tools/libxl/libxl_arm.c
+++ b/tools/libxl/libxl_arm.c
@@ -153,6 +153,18 @@ out:
     return rc;
 }
 
+unsigned long libxl__arch_get_required_paging_memory(unsigned long maxmem_kb,
+                                                     unsigned int smp_cpus)
+{
+    /*
+     * 256 pages (1MB) per vcpu,
+     * plus 1 page per MiB of RAM for the P2M map,
+     * This is higher than the minimum that Xen would allocate if no value
+     * were given (but the Xen minimum is for safety, not performance).
+     */
+    return 4 * (256 * smp_cpus + maxmem_kb / 1024);
+}
+
 static struct arch_info {
     const char *guest_type;
     const char *timer_compat;

diff --git a/tools/libxl/libxl_utils.c b/tools/libxl/libxl_utils.c
index b039143b8aef902725f732d54d3d804853ad3cd4..e18b1524ef7868dccd74f3f53c8564f3d5ac1736 100644 (file)
--- a/tools/libxl/libxl_utils.c
+++ b/tools/libxl/libxl_utils.c
@@ -18,6 +18,7 @@
 #include <ctype.h>
 
 #include "libxl_internal.h"
+#include "libxl_arch.h"
 #include "_paths.h"
 
 #ifndef LIBXL_HAVE_NONCONST_LIBXL_BASENAME_RETURN_VALUE
@@ -39,13 +40,7 @@ char *libxl_basename(const char *name)
 
 unsigned long libxl_get_required_shadow_memory(unsigned long maxmem_kb, unsigned int smp_cpus)
 {
-    /* 256 pages (1MB) per vcpu,
-       plus 1 page per MiB of RAM for the P2M map,
-       plus 1 page per MiB of RAM to shadow the resident processes.
-       This is higher than the minimum that Xen would allocate if no value
-       were given (but the Xen minimum is for safety, not performance).
-     */
-    return 4 * (256 * smp_cpus + 2 * (maxmem_kb / 1024));
+    return libxl__arch_get_required_paging_memory(maxmem_kb, smp_cpus);
 }
 
 char *libxl_domid_to_name(libxl_ctx *ctx, uint32_t domid)

diff --git a/tools/libxl/libxl_x86.c b/tools/libxl/libxl_x86.c
index f34c0edc1029fbf3cfc4f60759e49c13a3153093..348876e5c057902e849cca8804a7c988ea982de7 100644 (file)
--- a/tools/libxl/libxl_x86.c
+++ b/tools/libxl/libxl_x86.c
@@ -681,6 +681,18 @@ int libxl__arch_passthrough_mode_setdefault(libxl__gc *gc,
     return rc;
 }
 
+unsigned long libxl__arch_get_required_paging_memory(unsigned long maxmem_kb,
+                                                     unsigned int smp_cpus)
+{
+    /*
+     * 256 pages (1MB) per vcpu,
+     * plus 1 page per MiB of RAM for the P2M map,
+     * plus 1 page per MiB of RAM to shadow the resident processes.
+     * This is higher than the minimum that Xen would allocate if no value
+     * were given (but the Xen minimum is for safety, not performance).
+     */
+    return 4 * (256 * smp_cpus + 2 * (maxmem_kb / 1024));
+}
 
 /*
  * Local variables: