[Xen-changelog] [xen stable-4.7] x86: avoid #GP for PV guest MSR accesses

[patchbot@xxxxxxx]

commit 5805ab112bd9d509efcc6aa24c52085c5eb7ef71
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Oct 24 16:48:50 2017 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Oct 24 16:48:50 2017 +0200

    x86: avoid #GP for PV guest MSR accesses
    
    Halfway recent Linux kernels probe MISC_FEATURES_ENABLES on all CPUs,
    leading to ugly recovered #GP fault messages with debug builds on older
    systems. We can do better, so introduce synthetic feature flags for
    both this and PLATFORM_INFO to avoid the rdmsr_safe() altogether.
    
    Note that the r/o nature of PLATFORM_INFO is now also being enforced.
    
    The rdmsr_safe() uses for MISC_ENABLE are left in place as benign - it
    exists for all 64-bit capable Intel CPUs (see e.g. early_init_intel()).
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/cpu/intel.c         | 10 ++++++++--
 xen/arch/x86/traps.c             | 10 +++-------
 xen/include/asm-x86/cpufeature.h |  1 +
 3 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/xen/arch/x86/cpu/intel.c b/xen/arch/x86/cpu/intel.c
index 7b60aaa..181e815 100644
--- a/xen/arch/x86/cpu/intel.c
+++ b/xen/arch/x86/cpu/intel.c
@@ -22,8 +22,12 @@ static bool_t __init probe_intel_cpuid_faulting(void)
 {
        uint64_t x;
 
-       if (rdmsr_safe(MSR_INTEL_PLATFORM_INFO, x) ||
-           !(x & MSR_PLATFORM_INFO_CPUID_FAULTING))
+       if (rdmsr_safe(MSR_INTEL_PLATFORM_INFO, x))
+               return 0;
+
+       __set_bit(X86_FEATURE_MSR_PLATFORM_INFO, boot_cpu_data.x86_capability);
+
+       if (!(x & MSR_PLATFORM_INFO_CPUID_FAULTING))
                return 0;
 
        expected_levelling_cap |= LCAP_faulting;
@@ -321,6 +325,8 @@ static void early_init_intel(struct cpuinfo_x86 *c)
 
        if (test_bit(X86_FEATURE_CPUID_FAULTING, boot_cpu_data.x86_capability))
                __set_bit(X86_FEATURE_CPUID_FAULTING, c->x86_capability);
+       if (test_bit(X86_FEATURE_MSR_PLATFORM_INFO, 
boot_cpu_data.x86_capability))
+               __set_bit(X86_FEATURE_MSR_PLATFORM_INFO, c->x86_capability);
 
        intel_ctxt_switch_levelling(NULL);
 }
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 90b6071..3200850 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2866,11 +2866,8 @@ static int emulate_privileged_op(struct cpu_user_regs 
*regs)
             break;
 
         case MSR_INTEL_PLATFORM_INFO:
-            if ( boot_cpu_data.x86_vendor != X86_VENDOR_INTEL ||
-                 msr_content ||
-                 rdmsr_safe(MSR_INTEL_PLATFORM_INFO, msr_content) )
-                goto fail;
-            break;
+            /* The MSR is read-only. */
+            goto fail;
 
         case MSR_P6_PERFCTR(0)...MSR_P6_PERFCTR(7):
         case MSR_P6_EVNTSEL(0)...MSR_P6_EVNTSEL(3):
@@ -3001,8 +2998,7 @@ static int emulate_privileged_op(struct cpu_user_regs 
*regs)
             break;
 
         case MSR_INTEL_PLATFORM_INFO:
-            if ( boot_cpu_data.x86_vendor != X86_VENDOR_INTEL ||
-                 rdmsr_safe(MSR_INTEL_PLATFORM_INFO, val) )
+            if ( !boot_cpu_has(X86_FEATURE_MSR_PLATFORM_INFO) )
                 goto fail;
             regs->eax = regs->edx = 0;
             break;
diff --git a/xen/include/asm-x86/cpufeature.h b/xen/include/asm-x86/cpufeature.h
index 9c49206..1a11ebe 100644
--- a/xen/include/asm-x86/cpufeature.h
+++ b/xen/include/asm-x86/cpufeature.h
@@ -25,6 +25,7 @@
 #define X86_FEATURE_CPUID_FAULTING     ((FSCAPINTS+0)*32+ 6) /* cpuid faulting 
*/
 #define X86_FEATURE_CLFLUSH_MONITOR    ((FSCAPINTS+0)*32+ 7) /* clflush reqd 
with monitor */
 #define X86_FEATURE_APERFMPERF         ((FSCAPINTS+0)*32+ 8) /* APERFMPERF */
+#define X86_FEATURE_MSR_PLATFORM_INFO  ((FSCAPINTS+0)*32+ 9) /* PLATFORM_INFO 
MSR present */
 
 #define cpufeat_word(idx)      ((idx) / 32)
 #define cpufeat_bit(idx)       ((idx) % 32)
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.7

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