[PATCH v8 13/17] x86/boot: Calculate the most appropriate BTI mitigation to use

[Andrew Cooper <andrew.cooper3@citrix.com>]

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
v7:
 * static, and tweak comment
---
 docs/misc/xen-command-line.markdown |   6 ++-
 xen/arch/x86/spec_ctrl.c            | 104 ++++++++++++++++++++++++++++++++++--
 2 files changed, 105 insertions(+), 5 deletions(-)

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index b42abc6..1aa1225 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -246,7 +246,7 @@ enough. Setting this to a high value may cause boot failure, particularly if
 the NMI watchdog is also enabled.
 
 ### bti (x86)
-> `= List of [ thunk=retpoline|lfence|jmp ]`
+> `= List of [ thunk=retpoline|lfence|jmp, ibrs=<bool> ]`
 
 Branch Target Injection controls.  By default, Xen will pick the most
 appropriate BTI mitigations based on compiled in support, loaded microcode,
@@ -261,6 +261,10 @@ locations.  The default thunk is `retpoline` (generally preferred for Intel
 hardware), with the alternatives being `jmp` (a `jmp *%reg` gadget, minimal
 overhead), and `lfence` (an `lfence; jmp *%reg` gadget, preferred for AMD).
 
+On hardware supporting IBRS, the `ibrs=` option can be used to force or
+prevent Xen using the feature itself.  If Xen is not using IBRS itself,
+functionality is still set up so IBRS can be virtualised for guests.
+
 ### xenheap\_megabytes (arm32)
 > `= <size>`
 
diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 89e7287..7b0daaf 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -20,6 +20,7 @@
 #include <xen/init.h>
 #include <xen/lib.h>
 
+#include <asm/microcode.h>
 #include <asm/processor.h>
 #include <asm/spec_ctrl.h>
 
@@ -31,11 +32,12 @@ static enum ind_thunk {
     THUNK_LFENCE,
     THUNK_JMP,
 } opt_thunk __initdata = THUNK_DEFAULT;
+static int opt_ibrs __initdata = -1;
 
 static int __init parse_bti(const char *s)
 {
     const char *ss;
-    int rc = 0;
+    int val, rc = 0;
 
     do {
         ss = strchr(s, ',');
@@ -55,6 +57,8 @@ static int __init parse_bti(const char *s)
             else
                 rc = -EINVAL;
         }
+        else if ( (val = parse_boolean("ibrs", s, ss)) >= 0 )
+            opt_ibrs = val;
         else
             rc = -EINVAL;
 
@@ -91,24 +95,82 @@ static void __init print_details(enum ind_thunk thunk)
         printk(XENLOG_DEBUG "  Compiled-in support: INDIRECT_THUNK\n");
 
     printk(XENLOG_INFO
-           "BTI mitigations: Thunk %s\n",
+           "BTI mitigations: Thunk %s, Others:%s\n",
            thunk == THUNK_NONE      ? "N/A" :
            thunk == THUNK_RETPOLINE ? "RETPOLINE" :
            thunk == THUNK_LFENCE    ? "LFENCE" :
-           thunk == THUNK_JMP       ? "JMP" : "?");
+           thunk == THUNK_JMP       ? "JMP" : "?",
+           boot_cpu_has(X86_FEATURE_XEN_IBRS_SET)    ? " IBRS+" :
+           boot_cpu_has(X86_FEATURE_XEN_IBRS_CLEAR)  ? " IBRS-"      : "");
+}
+
+/* Calculate whether Retpoline is known-safe on this CPU. */
+static bool __init retpoline_safe(void)
+{
+    unsigned int ucode_rev = this_cpu(ucode_cpu_info).cpu_sig.rev;
+
+    if ( boot_cpu_data.x86_vendor == X86_VENDOR_AMD )
+        return true;
+
+    if ( boot_cpu_data.x86_vendor != X86_VENDOR_INTEL ||
+         boot_cpu_data.x86 != 6 )
+        return false;
+
+    switch ( boot_cpu_data.x86_model )
+    {
+    case 0x17: /* Penryn */
+    case 0x1d: /* Dunnington */
+    case 0x1e: /* Nehalem */
+    case 0x1f: /* Auburndale / Havendale */
+    case 0x1a: /* Nehalem EP */
+    case 0x2e: /* Nehalem EX */
+    case 0x25: /* Westmere */
+    case 0x2c: /* Westmere EP */
+    case 0x2f: /* Westmere EX */
+    case 0x2a: /* SandyBridge */
+    case 0x2d: /* SandyBridge EP/EX */
+    case 0x3a: /* IvyBridge */
+    case 0x3e: /* IvyBridge EP/EX */
+    case 0x3c: /* Haswell */
+    case 0x3f: /* Haswell EX/EP */
+    case 0x45: /* Haswell D */
+    case 0x46: /* Haswell H */
+        return true;
+
+        /*
+         * Broadwell processors are retpoline-safe after specific microcode
+         * versions.
+         */
+    case 0x3d: /* Broadwell */
+        return ucode_rev >= 0x28;
+    case 0x47: /* Broadwell H */
+        return ucode_rev >= 0x1b;
+    case 0x4f: /* Broadwell EP/EX */
+        return ucode_rev >= 0xb000025;
+    case 0x56: /* Broadwell D */
+        return false; /* TBD. */
+
+        /*
+         * Skylake and later processors are not retpoline-safe.
+         */
+    default:
+        return false;
+    }
 }
 
 void __init init_speculation_mitigations(void)
 {
     enum ind_thunk thunk = THUNK_DEFAULT;
+    bool ibrs = false;
 
     /*
      * Has the user specified any custom BTI mitigations?  If so, follow their
      * instructions exactly and disable all heuristics.
      */
-    if ( opt_thunk != THUNK_DEFAULT )
+    if ( opt_thunk != THUNK_DEFAULT || opt_ibrs != -1 )
     {
         thunk = opt_thunk;
+        ibrs  = !!opt_ibrs;
     }
     else
     {
@@ -124,7 +186,21 @@ void __init init_speculation_mitigations(void)
              */
             if ( cpu_has_lfence_dispatch )
                 thunk = THUNK_LFENCE;
+            /*
+             * On Intel hardware, we'd like to use retpoline in preference to
+             * IBRS, but only if it is safe on this hardware.
+             */
+            else if ( boot_cpu_has(X86_FEATURE_IBRSB) )
+            {
+                if ( retpoline_safe() )
+                    thunk = THUNK_RETPOLINE;
+                else
+                    ibrs = true;
+            }
         }
+        /* Without compiler thunk support, use IBRS if available. */
+        else if ( boot_cpu_has(X86_FEATURE_IBRSB) )
+            ibrs = true;
     }
 
     /*
@@ -135,6 +211,13 @@ void __init init_speculation_mitigations(void)
         thunk = THUNK_NONE;
 
     /*
+     * If IBRS is in use and thunks are compiled in, there is no point
+     * suffering extra overhead.  Switch to the least-overhead thunk.
+     */
+    if ( ibrs && thunk == THUNK_DEFAULT )
+        thunk = THUNK_JMP;
+
+    /*
      * If there are still no thunk preferences, the compiled default is
      * actually retpoline, and it is better than nothing.
      */
@@ -147,6 +230,19 @@ void __init init_speculation_mitigations(void)
     else if ( thunk == THUNK_JMP )
         setup_force_cpu_cap(X86_FEATURE_IND_THUNK_JMP);
 
+    if ( boot_cpu_has(X86_FEATURE_IBRSB) )
+    {
+        /*
+         * Even if we've chosen to not have IBRS set in Xen context, we still
+         * need the IBRS entry/exit logic to virtualise IBRS support for
+         * guests.
+         */
+        if ( ibrs )
+            setup_force_cpu_cap(X86_FEATURE_XEN_IBRS_SET);
+        else
+            setup_force_cpu_cap(X86_FEATURE_XEN_IBRS_CLEAR);
+    }
+
     print_details(thunk);
 }
 
-- 
2.1.4


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