[Xen-changelog] [xen stable-4.6] x86/msr: Correct the emulation behaviour of MSR_PRED_CMD

[patchbot@xxxxxxx]

commit 055abe41980b429eca9a899701f8327def937206
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri May 18 13:31:01 2018 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri May 18 13:31:01 2018 +0200

    x86/msr: Correct the emulation behaviour of MSR_PRED_CMD
    
    Experimentally, the behaviour of reserved bits in MSR_PRED_CMD changed 
between
    beta and production microcode, and now raises a #GP fault for set reserved
    bits.  The AMD spec for future hardware also specifies this behaviour, and 
it
    is the more sensible behaviour to implement.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    
    x86/msr: further correct the emulation behaviour of MSR_PRED_CMD
    
    Following commit a6aa678fa3 ("x86/msr: Correct the emulation behaviour
    of MSR_PRED_CMD") we may end up writing the low bit with the wrong
    value. While it's unlikely for a guest to want to write zero there, we
    should still permit (this without incurring the overhead of an actual
    barrier). Correcting this right away will also help whenever further
    bits in the MSR might become defined.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    master commit: a6aa678fa380e9369cc44701a181142322b3a4b0
    master date: 2018-04-16 13:18:19 +0100
    master commit: a996273d1fc10d14598985703227bfa35a91f681
    master date: 2018-04-18 11:16:37 +0200
---
 xen/arch/x86/hvm/hvm.c | 10 ++++------
 xen/arch/x86/traps.c   | 10 ++++------
 2 files changed, 8 insertions(+), 12 deletions(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index ebabc10763..78f44c5f66 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -5007,12 +5007,10 @@ int hvm_msr_write_intercept(unsigned int msr, uint64_t 
msr_content,
              !(ebx & cpufeat_mask(X86_FEATURE_IBPB)) )
             goto gp_fault; /* MSR available? */
 
-        /*
-         * The only defined behaviour is when writing PRED_CMD_IBPB.  In
-         * practice, real hardware accepts any value without faulting.
-         */
-        if ( msr_content & PRED_CMD_IBPB )
-            wrmsrl(MSR_PRED_CMD, PRED_CMD_IBPB);
+        if ( msr_content & ~PRED_CMD_IBPB )
+            goto gp_fault; /* Rsvd bit set? */
+
+        wrmsrl(MSR_PRED_CMD, msr_content);
         break;
 
     case MSR_ARCH_CAPABILITIES:
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index eab5a3f23f..c23f4c0edb 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2753,12 +2753,10 @@ static int emulate_privileged_op(struct cpu_user_regs 
*regs)
                  !(ebx & cpufeat_mask(X86_FEATURE_IBPB)) )
                 goto fail; /* MSR available? */
 
-            /*
-             * The only defined behaviour is when writing PRED_CMD_IBPB.  In
-             * practice, real hardware accepts any value without faulting.
-             */
-            if ( eax & PRED_CMD_IBPB )
-                wrmsrl(MSR_PRED_CMD, PRED_CMD_IBPB);
+            if ( msr_content & ~PRED_CMD_IBPB )
+                goto fail; /* Rsvd bit set? */
+
+            wrmsrl(MSR_PRED_CMD, msr_content);
             break;
 
         case MSR_P6_PERFCTR(0)...MSR_P6_PERFCTR(7):
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.6

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