[PATCH 4.9 11/31] kvm/x86: Export MDS_NO=0 to guests when TSX is enabled

Greg Kroah-Hartman
2019-11-15T06:22:05Z

From: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>

commit e1d38b63acd843cfdd4222bf19a26700fd5c699e upstream.

Export the IA32_ARCH_CAPABILITIES MSR bit MDS_NO=0 to guests on TSX
Async Abort(TAA) affected hosts that have TSX enabled and updated
microcode. This is required so that the guests don't complain,

"Vulnerable: Clear CPU buffers attempted, no microcode"

when the host has the updated microcode to clear CPU buffers.

Microcode update also adds support for MSR_IA32_TSX_CTRL which is
enumerated by the ARCH_CAP_TSX_CTRL bit in IA32_ARCH_CAPABILITIES MSR.
Guests can't do this check themselves when the ARCH_CAP_TSX_CTRL bit is
not exported to the guests.

In this case export MDS_NO=0 to the guests. When guests have
CPUID.MD_CLEAR=1, they deploy MDS mitigation which also mitigates TAA.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Neelima Krishnan <neelima.krishnan@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/kvm/x86.c | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1050,6 +1050,25 @@ u64 kvm_get_arch_capabilities(void)
if (!boot_cpu_has_bug(X86_BUG_MDS))
data |= ARCH_CAP_MDS_NO;

+ /*
+ * On TAA affected systems, export MDS_NO=0 when:
+ * - TSX is enabled on the host, i.e. X86_FEATURE_RTM=1.
+ * - Updated microcode is present. This is detected by
+ * the presence of ARCH_CAP_TSX_CTRL_MSR and ensures
+ * that VERW clears CPU buffers.
+ *
+ * When MDS_NO=0 is exported, guests deploy clear CPU buffer
+ * mitigation and don't complain:
+ *
+ * "Vulnerable: Clear CPU buffers attempted, no microcode"
+ *
+ * If TSX is disabled on the system, guests are also mitigated against
+ * TAA and clear CPU buffer mitigation is not required for guests.
+ */
+ if (boot_cpu_has_bug(X86_BUG_TAA) && boot_cpu_has(X86_FEATURE_RTM) &&
+ (data & ARCH_CAP_TSX_CTRL_MSR))
+ data &= ~ARCH_CAP_MDS_NO;
+
return data;
}