A single fix for x86 which removes the RDPID usage from the paranoid entry

path and unconditionally uses LSL to retrieve the CPU number. RDPID depends
 on MSR_TSX_AUX.  KVM has an optmization to avoid expensive MRS read/writes
 on VMENTER/EXIT. It caches the MSR values and restores them either when
 leaving the run loop, on preemption or when going out to user
 space. MSR_TSX_AUX is part of that lazy MSR set, so after writing the guest
 value and before the lazy restore any exception using the paranoid entry
 will read the guest value and use it as CPU number to retrieve the GSBASE
 value for the current CPU when FSGSBASE is enabled. As RDPID is only used
 in that particular entry path, there is no reason to burden VMENTER/EXIT
 with two extra MSR writes. Remove the RDPID optimization, which is not even
 backed by numbers from the paranoid entry path instead.
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Merge tag 'x86-urgent-2020-08-23' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 fix from Thomas Gleixner:
 "A single fix for x86 which removes the RDPID usage from the paranoid
  entry path and unconditionally uses LSL to retrieve the CPU number.

  RDPID depends on MSR_TSX_AUX. KVM has an optmization to avoid
  expensive MRS read/writes on VMENTER/EXIT. It caches the MSR values
  and restores them either when leaving the run loop, on preemption or
  when going out to user space. MSR_TSX_AUX is part of that lazy MSR
  set, so after writing the guest value and before the lazy restore any
  exception using the paranoid entry will read the guest value and use
  it as CPU number to retrieve the GSBASE value for the current CPU when
  FSGSBASE is enabled. As RDPID is only used in that particular entry
  path, there is no reason to burden VMENTER/EXIT with two extra MSR
  writes. Remove the RDPID optimization, which is not even backed by
  numbers from the paranoid entry path instead"

* tag 'x86-urgent-2020-08-23' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/entry/64: Do not use RDPID in paranoid entry to accomodate KVM

arch/x86/entry/calling.h

@@ -374,12 +374,14 @@ For 32-bit we have the following conventions - kernel is built with
  * Fetch the per-CPU GSBASE value for this processor and put it in @reg.
  * We normally use %gs for accessing per-CPU data, but we are setting up
  * %gs here and obviously can not use %gs itself to access per-CPU data.
+ *
+ * Do not use RDPID, because KVM loads guest's TSC_AUX on vm-entry and
+ * may not restore the host's value until the CPU returns to userspace.
+ * Thus the kernel would consume a guest's TSC_AUX if an NMI arrives
+ * while running KVM's run loop.
  */
 .macro GET_PERCPU_BASE reg:req
-	ALTERNATIVE \
-		"LOAD_CPU_AND_NODE_SEG_LIMIT \reg", \
-		"RDPID	\reg", \
-		X86_FEATURE_RDPID
+	LOAD_CPU_AND_NODE_SEG_LIMIT \reg
 	andq	$VDSO_CPUNODE_MASK, \reg
 	movq	__per_cpu_offset(, \reg, 8), \reg
 .endm