[PATCH v9 2/2] x86/mm: Don't disable PCID if the kernel is running on a hypervisor

[Xi Ruoyao]

The Intel erratum for "incomplete Global INVLPG flushes" says:

    This erratum does not apply in VMX non-root operation. It applies
    only when PCIDs are enabled and either in VMX root operation or
    outside VMX operation.

So if the kernel is running in a hypervisor, we are in VMX non-root
operation and we should be safe to use PCID.

Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Michael Kelley <mhklinux@xxxxxxxxxxx>
Cc: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Link: https://lore.kernel.org/all/168436059559.404.13934972543631851306.tip-bot2@tip-bot2/
Link: https://cdrdv2.intel.com/v1/dl/getContent/740518 # RPL042, rev. 13
Link: https://cdrdv2.intel.com/v1/dl/getContent/682436 # ADL063, rev. 24
Signed-off-by: Xi Ruoyao <xry111@xxxxxxxxxxx>
---
 arch/x86/mm/init.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index c318cdc35467..b20e453c1217 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -286,7 +286,7 @@ static const struct x86_cpu_id invlpg_miss_ids[] = {
 
 static void setup_pcid(void)
 {
-	const struct x86_cpu_id *invlpg_miss_match;
+	const struct x86_cpu_id *invlpg_miss_match = NULL;
 
 	if (!IS_ENABLED(CONFIG_X86_64))
 		return;
@@ -294,7 +294,9 @@ static void setup_pcid(void)
 	if (!boot_cpu_has(X86_FEATURE_PCID))
 		return;
 
-	invlpg_miss_match = x86_match_cpu(invlpg_miss_ids);
+	/* Only bare-metal is affected.  PCIDs in guests are OK.  */
+	if (!boot_cpu_has(X86_FEATURE_HYPERVISOR))
+		invlpg_miss_match = x86_match_cpu(invlpg_miss_ids);
 
 	if (invlpg_miss_match &&
 	    boot_cpu_data.microcode < invlpg_miss_match->driver_data) {
-- 
2.44.0