This property is required for the AMD's recommended mitigation for Branch
Target Injection, but Xen needs to cope with being unable to detect or modify
the MSR.
Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com>
---
v4:
* New
v5:
* Use mnemonics.
---
xen/arch/x86/cpu/amd.c | 35 ++++++++++++++++++++++++++++++++++-
xen/include/asm-x86/cpufeature.h | 1 +
xen/include/asm-x86/cpufeatures.h | 1 +
xen/include/asm-x86/msr-index.h | 1 +
4 files changed, 37 insertions(+), 1 deletion(-)
diff --git a/xen/arch/x86/cpu/amd.c b/xen/arch/x86/cpu/amd.c
index 5f36ac7..db78b50 100644
--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -558,8 +558,41 @@ static void init_amd(struct cpuinfo_x86 *c)
wrmsr_amd_safe(0xc001100d, l, h & ~1);
}
+ /*
+ * Attempt to set lfence to be Dispatch Serialising. This MSR almost
+ * certainly isn't virtualised (and Xen at least will leak the real
+ * value in but silently discard writes), as well as being per-core
+ * rather than per-thread, so do a full safe read/write/readback cycle
+ * in the worst case.
+ */
+ if (c->x86 == 0x0f || c->x86 == 0x11)
+ /* Always dispatch serialising on this hardare. */
+ __set_bit(X86_FEATURE_LFENCE_DISPATCH, c->x86_capability);
+ else if (c->x86 == 0x10 || c->x86 >= 0x12) {
+ if (rdmsr_safe(MSR_AMD64_DE_CFG, value))
+ /* Unable to read. Assume the safer default. */
+ __clear_bit(X86_FEATURE_LFENCE_DISPATCH,
+ c->x86_capability);
+ else if (value & AMD64_DE_CFG_LFENCE_SERIALISE)
+ /* Already dispatch serialising. */
+ __set_bit(X86_FEATURE_LFENCE_DISPATCH,
+ c->x86_capability);
+ else if (wrmsr_safe(MSR_AMD64_DE_CFG,
+ value | AMD64_DE_CFG_LFENCE_SERIALISE) ||
+ rdmsr_safe(MSR_AMD64_DE_CFG, value) ||
+ !(value & AMD64_DE_CFG_LFENCE_SERIALISE))
+ /* Attempt to set failed. Assume the safer default */
+ __clear_bit(X86_FEATURE_LFENCE_DISPATCH,
+ c->x86_capability);
+ else
+ /* Successfully enabled! */
+ __set_bit(X86_FEATURE_LFENCE_DISPATCH,
+ c->x86_capability);
+ }
+
/* MFENCE stops RDTSC speculation */
- __set_bit(X86_FEATURE_MFENCE_RDTSC, c->x86_capability);
+ if (!cpu_has_lfence_dispatch)
+ __set_bit(X86_FEATURE_MFENCE_RDTSC, c->x86_capability);
switch(c->x86)
{
diff --git a/xen/include/asm-x86/cpufeature.h b/xen/include/asm-x86/cpufeature.h
index 84cc51d..adc333f 100644
--- a/xen/include/asm-x86/cpufeature.h
+++ b/xen/include/asm-x86/cpufeature.h
@@ -104,6 +104,7 @@
#define cpu_has_arch_perfmon boot_cpu_has(X86_FEATURE_ARCH_PERFMON)
#define cpu_has_cpuid_faulting boot_cpu_has(X86_FEATURE_CPUID_FAULTING)
#define cpu_has_aperfmperf boot_cpu_has(X86_FEATURE_APERFMPERF)
+#define cpu_has_lfence_dispatch boot_cpu_has(X86_FEATURE_LFENCE_DISPATCH)
enum _cache_type {
CACHE_TYPE_NULL = 0,
diff --git a/xen/include/asm-x86/cpufeatures.h
b/xen/include/asm-x86/cpufeatures.h
index bc98227..58b37d6 100644
--- a/xen/include/asm-x86/cpufeatures.h
+++ b/xen/include/asm-x86/cpufeatures.h
@@ -22,3 +22,4 @@ XEN_CPUFEATURE(APERFMPERF, (FSCAPINTS+0)*32+ 8) /*
APERFMPERF */
XEN_CPUFEATURE(MFENCE_RDTSC, (FSCAPINTS+0)*32+ 9) /* MFENCE synchronizes
RDTSC */
XEN_CPUFEATURE(XEN_SMEP, (FSCAPINTS+0)*32+10) /* SMEP gets used by Xen
itself */
XEN_CPUFEATURE(XEN_SMAP, (FSCAPINTS+0)*32+11) /* SMAP gets used by Xen
itself */
+XEN_CPUFEATURE(LFENCE_DISPATCH, (FSCAPINTS+0)*32+12) /* lfence set as Dispatch
Serialising */
diff --git a/xen/include/asm-x86/msr-index.h b/xen/include/asm-x86/msr-index.h
index a834f3b..56f5359 100644
--- a/xen/include/asm-x86/msr-index.h
+++ b/xen/include/asm-x86/msr-index.h
@@ -207,6 +207,7 @@
#define MSR_AMD64_IC_CFG 0xc0011021
#define MSR_AMD64_DC_CFG 0xc0011022
#define MSR_AMD64_DE_CFG 0xc0011029
+#define AMD64_DE_CFG_LFENCE_SERIALISE (_AC(1, ULL) << 1)
#define MSR_AMD64_DR0_ADDRESS_MASK 0xc0011027
#define MSR_AMD64_DR1_ADDRESS_MASK 0xc0011019
--
2.1.4
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