[RFC 10/11] x86/mm: Enable CR4.PCIDE on supported systems

[Andy Lutomirski <luto@kernel.org>]

We can use PCID if the CPU has PCID and PGE and we're not on Xen.

By itself, this has no effect.  The next patch will start using
PCID.

Cc: Juergen Gross <jgross@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/include/asm/tlbflush.h |  8 ++++++++
 arch/x86/kernel/cpu/common.c    | 15 +++++++++++++++
 arch/x86/xen/setup.c            |  6 ++++++
 3 files changed, 29 insertions(+)

diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index 87b13e51e867..57b305e13c4c 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -243,6 +243,14 @@ static inline void __flush_tlb_all(void)
 		__flush_tlb_global();
 	else
 		__flush_tlb();
+
+	/*
+	 * Note: if we somehow had PCID but not PGE, then this wouldn't work --
+	 * we'd end up flushing kernel translations for the current ASID but
+	 * we might fail to flush kernel translations for other cached ASIDs.
+	 *
+	 * To avoid this issue, we force PCID off if PGE is off.
+	 */
 }
 
 static inline void __flush_tlb_one(unsigned long addr)
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 904485e7b230..01caf66b270f 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1143,6 +1143,21 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	setup_smep(c);
 	setup_smap(c);
 
+	/* Set up PCID */
+	if (cpu_has(c, X86_FEATURE_PCID)) {
+		if (cpu_has(c, X86_FEATURE_PGE)) {
+			cr4_set_bits(X86_CR4_PCIDE);
+		} else {
+			/*
+			 * flush_tlb_all(), as currently implemented, won't
+			 * work if PCID is on but PGE is not.  Since that
+			 * combination doesn't exist on real hardware, there's
+			 * no reason to try to fully support it.
+			 */
+			clear_cpu_cap(c, X86_FEATURE_PCID);
+		}
+	}
+
 	/*
 	 * The vendor-specific functions might have changed features.
 	 * Now we do "generic changes."
diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index a5bf7c451435..7681202b2857 100644
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -1037,6 +1037,12 @@ void __init xen_arch_setup(void)
 	}
 #endif
 
+	/*
+	 * Xen would need some work to support PCID: CR3 handling as well
+	 * as xen_flush_tlb_others() would need updating.
+	 */
+	setup_clear_cpu_cap(X86_FEATURE_PCID);
+
 	memcpy(boot_command_line, xen_start_info->cmd_line,
 	       MAX_GUEST_CMDLINE > COMMAND_LINE_SIZE ?
 	       COMMAND_LINE_SIZE : MAX_GUEST_CMDLINE);
-- 
2.9.3

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