[Xen-changelog] [xen stable-4.10] x86/msr: Correct the emulation behaviour of MSR_PRED_CMD

[patchbot@xxxxxxx]

commit 656c14780c5c69ed8289b1f41fcdf1c84446bbba
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Wed Apr 18 16:34:37 2018 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Apr 18 16:34:37 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/msr.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/msr.c b/xen/arch/x86/msr.c
index dc5aeb30da..48d061d7da 100644
--- a/xen/arch/x86/msr.c
+++ b/xen/arch/x86/msr.c
@@ -224,12 +224,11 @@ int guest_wrmsr(struct vcpu *v, uint32_t msr, uint64_t 
val)
         if ( !cp->feat.ibrsb && !cp->extd.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 ( v == curr && (val & PRED_CMD_IBPB) )
-            wrmsrl(MSR_PRED_CMD, PRED_CMD_IBPB);
+        if ( val & ~PRED_CMD_IBPB )
+            goto gp_fault; /* Rsvd bit set? */
+
+        if ( v == curr )
+            wrmsrl(MSR_PRED_CMD, val);
         break;
 
     case MSR_INTEL_MISC_FEATURES_ENABLES:
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.10

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