[MODERATED] [PATCH 6/9] TAA v2 6

From: Pawan Gupta <[email protected]>
Subject: [PATCH 6/9] kvm/x86: Export MDS_NO=0 to guests when TSX is enabled

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 <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Reviewed-by: Tony Luck <[email protected]>
Reviewed-by: Josh Poimboeuf <[email protected]>
Tested-by: Neelima Krishnan <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Joerg Roedel <[email protected]>
Cc: kvm ML <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: "Radim Krcmar" <[email protected]>
Cc: Sean Christopherson <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: x86-ml <[email protected]>
---
 arch/x86/kvm/x86.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 661e2bf38526..fb8d80caf642 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1299,6 +1299,25 @@ static 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;
 }
 
-- 
2.21.0