[3.16.y-ckt stable] Patch "x86: kvm: use alternatives for VMCALL vs. VMMCALL if kernel text is read-only" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    x86: kvm: use alternatives for VMCALL vs. VMMCALL if kernel text is read-only

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt3.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From e8af13a8692cd9b52b1b148b3b6b79828bca1208 Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini at redhat.com>
Date: Mon, 22 Sep 2014 13:17:48 +0200
Subject: x86: kvm: use alternatives for VMCALL vs. VMMCALL if kernel text is
 read-only

commit c1118b3602c2329671ad5ec8bdf8e374323d6343 upstream.

On x86_64, kernel text mappings are mapped read-only with CONFIG_DEBUG_RODATA.
In that case, KVM will fail to patch VMCALL instructions to VMMCALL
as required on AMD processors.

The failure mode is currently a divide-by-zero exception, which obviously
is a KVM bug that has to be fixed.  However, picking the right instruction
between VMCALL and VMMCALL will be faster and will help if you cannot upgrade
the hypervisor.

Reported-by: Chris Webb <chris at arachsys.com>
Tested-by: Chris Webb <chris at arachsys.com>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Ingo Molnar <mingo at redhat.com>
Cc: "H. Peter Anvin" <hpa at zytor.com>
Cc: x86 at kernel.org
Acked-by: Borislav Petkov <bp at suse.de>
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
Cc: Chris J Arges <chris.j.arges at canonical.com>
[ luis: used Chris backport to 3.16 ]
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 arch/x86/include/asm/cpufeature.h |  1 +
 arch/x86/include/asm/kvm_para.h   | 10 ++++++++--
 arch/x86/kernel/cpu/amd.c         |  7 +++++++
 3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index e265ff95d16d..51f75503541d 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -203,6 +203,7 @@
 #define X86_FEATURE_DECODEASSISTS (8*32+12) /* AMD Decode Assists support */
 #define X86_FEATURE_PAUSEFILTER (8*32+13) /* AMD filtered pause intercept */
 #define X86_FEATURE_PFTHRESHOLD (8*32+14) /* AMD pause filter threshold */
+#define X86_FEATURE_VMMCALL     ( 8*32+15) /* Prefer vmmcall to vmcall */


 /* Intel-defined CPU features, CPUID level 0x00000007:0 (ebx), word 9 */
diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
index c7678e43465b..e62cf897f781 100644
--- a/arch/x86/include/asm/kvm_para.h
+++ b/arch/x86/include/asm/kvm_para.h
@@ -2,6 +2,7 @@
 #define _ASM_X86_KVM_PARA_H

 #include <asm/processor.h>
+#include <asm/alternative.h>
 #include <uapi/asm/kvm_para.h>

 extern void kvmclock_init(void);
@@ -16,10 +17,15 @@ static inline bool kvm_check_and_clear_guest_paused(void)
 }
 #endif /* CONFIG_KVM_GUEST */

-/* This instruction is vmcall.  On non-VT architectures, it will generate a
- * trap that we will then rewrite to the appropriate instruction.
+#ifdef CONFIG_DEBUG_RODATA
+#define KVM_HYPERCALL \
+        ALTERNATIVE(".byte 0x0f,0x01,0xc1", ".byte 0x0f,0x01,0xd9", X86_FEATURE_VMMCALL)
+#else
+/* On AMD processors, vmcall will generate a trap that we will
+ * then rewrite to the appropriate instruction.
  */
 #define KVM_HYPERCALL ".byte 0x0f,0x01,0xc1"
+#endif

 /* For KVM hypercalls, a three-byte sequence of either the vmcall or the vmmcall
  * instruction.  The hypervisor may replace it with something else but only the
diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index ce8b8ff0e0ef..edefacacc2cc 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -506,6 +506,13 @@ static void early_init_amd(struct cpuinfo_x86 *c)
 	}
 #endif

+	/*
+	 * This is only needed to tell the kernel whether to use VMCALL
+	 * and VMMCALL.  VMMCALL is never executed except under virt, so
+	 * we can set it unconditionally.
+	 */
+	set_cpu_cap(c, X86_FEATURE_VMMCALL);
+
 	/* F16h erratum 793, CVE-2013-6885 */
 	if (c->x86 == 0x16 && c->x86_model <= 0xf)
 		msr_set_bit(MSR_AMD64_LS_CFG, 15);
--
2.1.0