x86/boot: Report details of speculative mitigations

Nothing very interesting at the moment, but the logic will grow as new
mitigations are added.

This is part of XSA-254.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
master commit: 31d6c53adf
master date: 2018-01-16 17:45:50 +0000

xen/arch/x86/Makefile

@@ -56,6 +56,7 @@ obj-y += setup.o
 obj-y += shutdown.o
 obj-y += smp.o
 obj-y += smpboot.o
+obj-y += spec_ctrl.o
 obj-y += srat.o
 obj-y += string.o
 obj-y += sysctl.o

xen/arch/x86/setup.c

@@ -52,6 +52,7 @@
 #include <asm/alternative.h>
 #include <asm/mc146818rtc.h>
 #include <asm/cpuid.h>
+#include <asm/spec_ctrl.h>
 
 /* opt_nosmp: If true, secondary processors are ignored. */
 static bool_t __initdata opt_nosmp;
@@ -1479,6 +1480,8 @@ void __init noreturn __start_xen(unsigned long mbi_p)
     if ( cpu_has_fsgsbase )
         set_in_cr4(X86_CR4_FSGSBASE);
 
+    init_speculation_mitigations();
+
     init_idle_domain();
 
     this_cpu(stubs.addr) = alloc_stub_page(smp_processor_id(),

xen/arch/x86/spec_ctrl.c

@@ -0,0 +1,75 @@
+/******************************************************************************
+ * arch/x86/spec_ctrl.c
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Copyright (c) 2017-2018 Citrix Systems Ltd.
+ */
+#include <xen/init.h>
+#include <xen/lib.h>
+
+#include <asm/processor.h>
+#include <asm/spec_ctrl.h>
+
+enum ind_thunk {
+    THUNK_DEFAULT, /* Decide which thunk to use at boot time. */
+    THUNK_NONE,    /* Missing compiler support for thunks. */
+
+    THUNK_RETPOLINE,
+};
+
+static void __init print_details(enum ind_thunk thunk)
+{
+    printk(XENLOG_DEBUG "Speculative mitigation facilities:\n");
+
+    /* Compiled-in support which pertains to BTI mitigations. */
+    if ( IS_ENABLED(CONFIG_INDIRECT_THUNK) )
+        printk(XENLOG_DEBUG "  Compiled-in support: INDIRECT_THUNK\n");
+
+    printk(XENLOG_INFO
+           "BTI mitigations: Thunk %s\n",
+           thunk == THUNK_NONE      ? "N/A" :
+           thunk == THUNK_RETPOLINE ? "RETPOLINE" : "?");
+}
+
+void __init init_speculation_mitigations(void)
+{
+    enum ind_thunk thunk = THUNK_DEFAULT;
+
+    /*
+     * Supplimentary minor adjustments.  Without compiler support, there are
+     * no thunks.
+     */
+    if ( !IS_ENABLED(CONFIG_INDIRECT_THUNK) )
+        thunk = THUNK_NONE;
+
+    /*
+     * If there are still no thunk preferences, the compiled default is
+     * actually retpoline, and it is better than nothing.
+     */
+    if ( thunk == THUNK_DEFAULT )
+        thunk = THUNK_RETPOLINE;
+
+    print_details(thunk);
+}
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * tab-width: 4
+ * indent-tabs-mode: nil
+ * End:
+ */

xen/include/asm-x86/spec_ctrl.h

@@ -0,0 +1,35 @@
+/******************************************************************************
+ * include/asm-x86/spec_ctrl.h
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Copyright (c) 2017-2018 Citrix Systems Ltd.
+ */
+
+#ifndef __X86_SPEC_CTRL_H__
+#define __X86_SPEC_CTRL_H__
+
+void init_speculation_mitigations(void);
+
+#endif /* !__X86_SPEC_CTRL_H__ */
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * tab-width: 4
+ * indent-tabs-mode: nil
+ * End:
+ */