[PATCH v2 2/5] xen/version: Calculate xen_capabilities_info once at boot

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The arch_get_xen_caps() infrastructure is horribly inefficient, for something
that is constant after features have been resolved on boot.

Every instance used snprintf() to format constants into a string (which gets
shorter when %d gets resolved!), which gets double buffered on the stack.

Switch to using string literals with the "3.0" inserted - these numbers
haven't changed in 18 years (The Xen 3.0 release was Dec 5th 2005).

Use initcalls to format the data into xen_cap_info, which is deliberately not
of type xen_capabilities_info_t because a 1k array is a silly overhead for
storing a maximum of 77 chars (the x86 version) and isn't liable to need any
more space in the forseeable future.

This speeds up the the XENVER_capabilities hypercall, but the purpose of the
change is to allow us to introduce a better XENVER_* API that doesn't force us
to put a 1k buffer on the stack.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Julien Grall <julien@xxxxxxx>
CC: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
CC: Bertrand Marquis <bertrand.marquis@xxxxxxx>

v2:
 * New

If Xen had strncpy(), then the hunk in do_xen_version() could read:

  if ( deny )
     memset(info, 0, sizeof(info));
  else
     strncpy(info, xen_cap_info, sizeof(info));

to avoid double processing the start of the buffer, but given the ABI (must
write 1k chars into the guest), I cannot see any way of taking info off the
stack without some kind of strncpy_to_guest() API.

Moving to __initcall() also allows new architectures to not implement this
API, and I'm going to recommend strongly that they dont.  Its a very dubious
way of signalling about 3 bits of info to the toolstack, and inefficient to
use (the toolstack has to do string parsing on the result figure out if
PV64/PV32/HVM is available).
---
 xen/arch/arm/setup.c        | 19 ++++++-------------
 xen/arch/x86/setup.c        | 31 ++++++++++---------------------
 xen/common/kernel.c         |  3 ++-
 xen/include/xen/hypercall.h |  2 --
 xen/include/xen/version.h   |  2 ++
 5 files changed, 20 insertions(+), 37 deletions(-)

diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
index 1f26f67b90e3..b71b4bc506e0 100644
--- a/xen/arch/arm/setup.c
+++ b/xen/arch/arm/setup.c
@@ -1194,24 +1194,17 @@ void __init start_xen(unsigned long boot_phys_offset,
     switch_stack_and_jump(idle_vcpu[0]->arch.cpu_info, init_done);
 }
 
-void arch_get_xen_caps(xen_capabilities_info_t *info)
+static int __init init_xen_cap_info(void)
 {
-    /* Interface name is always xen-3.0-* for Xen-3.x. */
-    int major = 3, minor = 0;
-    char s[32];
-
-    (*info)[0] = '\0';
-
 #ifdef CONFIG_ARM_64
-    snprintf(s, sizeof(s), "xen-%d.%d-aarch64 ", major, minor);
-    safe_strcat(*info, s);
+    safe_strcat(xen_cap_info, "xen-3.0-aarch64 ");
 #endif
     if ( cpu_has_aarch32 )
-    {
-        snprintf(s, sizeof(s), "xen-%d.%d-armv7l ", major, minor);
-        safe_strcat(*info, s);
-    }
+        safe_strcat(xen_cap_info, "xen-3.0-armv7l ");
+
+    return 0;
 }
+__initcall(init_xen_cap_info);
 
 /*
  * Local variables:
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 566422600d94..f80821469ece 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1956,35 +1956,24 @@ void __init noreturn __start_xen(unsigned long mbi_p)
     unreachable();
 }
 
-void arch_get_xen_caps(xen_capabilities_info_t *info)
+static int __init cf_check init_xen_cap_info(void)
 {
-    /* Interface name is always xen-3.0-* for Xen-3.x. */
-    int major = 3, minor = 0;
-    char s[32];
-
-    (*info)[0] = '\0';
-
     if ( IS_ENABLED(CONFIG_PV) )
     {
-        snprintf(s, sizeof(s), "xen-%d.%d-x86_64 ", major, minor);
-        safe_strcat(*info, s);
+        safe_strcat(xen_cap_info, "xen-3.0-x86_64 ");
 
         if ( opt_pv32 )
-        {
-            snprintf(s, sizeof(s), "xen-%d.%d-x86_32p ", major, minor);
-            safe_strcat(*info, s);
-        }
+            safe_strcat(xen_cap_info, "xen-3.0-x86_32p ");
     }
     if ( hvm_enabled )
-    {
-        snprintf(s, sizeof(s), "hvm-%d.%d-x86_32 ", major, minor);
-        safe_strcat(*info, s);
-        snprintf(s, sizeof(s), "hvm-%d.%d-x86_32p ", major, minor);
-        safe_strcat(*info, s);
-        snprintf(s, sizeof(s), "hvm-%d.%d-x86_64 ", major, minor);
-        safe_strcat(*info, s);
-    }
+        safe_strcat(xen_cap_info,
+                    "hvm-3.0-x86_32 "
+                    "hvm-3.0-x86_32p "
+                    "hvm-3.0-x86_64 ");
+
+    return 0;
 }
+__initcall(init_xen_cap_info);
 
 int __hwdom_init xen_in_range(unsigned long mfn)
 {
diff --git a/xen/common/kernel.c b/xen/common/kernel.c
index f7b1f65f373c..4fa1d6710115 100644
--- a/xen/common/kernel.c
+++ b/xen/common/kernel.c
@@ -30,6 +30,7 @@ enum system_state system_state = SYS_STATE_early_boot;
 
 xen_commandline_t saved_cmdline;
 static const char __initconst opt_builtin_cmdline[] = CONFIG_CMDLINE;
+char __ro_after_init xen_cap_info[128];
 
 static int assign_integer_param(const struct kernel_param *param, uint64_t val)
 {
@@ -509,7 +510,7 @@ long do_xen_version(int cmd, XEN_GUEST_HANDLE_PARAM(void) 
arg)
 
         memset(info, 0, sizeof(info));
         if ( !deny )
-            arch_get_xen_caps(&info);
+            safe_strcpy(info, xen_cap_info);
 
         if ( copy_to_guest(arg, info, ARRAY_SIZE(info)) )
             return -EFAULT;
diff --git a/xen/include/xen/hypercall.h b/xen/include/xen/hypercall.h
index f307dfb59760..15b6be6ec818 100644
--- a/xen/include/xen/hypercall.h
+++ b/xen/include/xen/hypercall.h
@@ -56,6 +56,4 @@ common_vcpu_op(int cmd,
     struct vcpu *v,
     XEN_GUEST_HANDLE_PARAM(void) arg);
 
-void arch_get_xen_caps(xen_capabilities_info_t *info);
-
 #endif /* __XEN_HYPERCALL_H__ */
diff --git a/xen/include/xen/version.h b/xen/include/xen/version.h
index 93c58773630c..4856ad1b446d 100644
--- a/xen/include/xen/version.h
+++ b/xen/include/xen/version.h
@@ -19,6 +19,8 @@ const char *xen_deny(void);
 const char *xen_build_info(void);
 int xen_build_id(const void **p, unsigned int *len);
 
+extern char xen_cap_info[128];
+
 #ifdef BUILD_ID
 void xen_build_init(void);
 int xen_build_id_check(const Elf_Note *n, unsigned int n_sz,
-- 
2.11.0