[PATCH v2] x86/spectre_v2: Don't check bad microcode versions when running under hypervisors.

[Konrad Rzeszutek Wilk]

As:
 1) We know they lie about the env anyhow (host mismatch)
 2) Even if the hypervisor (Xen, KVM, VMWare, etc) provided
    a valid "correct" value, it all gets to be very murky
    when migration happens (do you provide the "new"
    microcode of the machine?).

And in reality the cloud vendors are the ones that should make
sure that the microcode that is running is correct and we should
just sing lalalala and trust them.

CC: stable@xxxxxxxxxxxxxxx
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
CC: x86@xxxxxxxxxx
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

---
v2: Change comments to be more in line with the state of the world.
v3: Use cpu_has instead of boot_cpu_has per Borislav's review.
---
 arch/x86/kernel/cpu/intel.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index d19e903214b4..4aa9fd379390 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -144,6 +144,13 @@ static bool bad_spectre_microcode(struct cpuinfo_x86 *c)
 {
 	int i;
 
+	/*
+	 * We know that the hypervisor lie to us on the microcode version so
+	 * we may as well hope that it is running the correct version.
+	 */
+	if (cpu_has(c, X86_FEATURE_HYPERVISOR))
+		return false;
+
 	for (i = 0; i < ARRAY_SIZE(spectre_bad_microcodes); i++) {
 		if (c->x86_model == spectre_bad_microcodes[i].model &&
 		    c->x86_stepping == spectre_bad_microcodes[i].stepping)
-- 
2.13.4