linux-kernel - [PATCH v2] x86: Skip WBINVD instruction for VM guest

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Date:   Thu, 18 Nov 2021 20:03:30 -0800
From:   Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "Rafael J . Wysocki" <rjw@...ysocki.net>
Cc:     "H . Peter Anvin" <hpa@...or.com>, Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org
Subject: [PATCH v2] x86: Skip WBINVD instruction for VM guest

ACPI mandates that CPU caches be flushed before entering any sleep
state. This ensures that the CPU and its caches can be powered down
without losing data.

ACPI-based VMs have maintained this sleep-state-entry behavior.
However, cache flushing for VM sleep state entry is useless. Unlike on
bare metal, guest sleep states are not correlated with potential data
loss of any kind; the host is responsible for data preservation. In
fact, some KVM configurations simply skip the cache flushing
instruction (see need_emulate_wbinvd()).

Further, on TDX systems, the WBINVD instruction causes an
unconditional #VE exception.  If this cache flushing remained, it would
need extra code in the form of a #VE handler.

All use of ACPI_FLUSH_CPU_CACHE() appears to be in sleep-state-related
code.

This means that the ACPI use of WBINVD is at *best* superfluous.

Disable ACPI CPU cache flushing on all X86_FEATURE_HYPERVISOR systems,
which includes TDX.

Cc: Rafael J. Wysocki <rjw@...ysocki.net>
Cc: linux-acpi@...r.kernel.org
Reviewed-by: Dan Williams <dan.j.williams@...el.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
---

Changes since v1:
 * Used cpu_feature_enabled() instead of boot_cpu_has().

 arch/x86/include/asm/acenv.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/acenv.h b/arch/x86/include/asm/acenv.h
index 9aff97f0de7f..dba05c74bd7e 100644
--- a/arch/x86/include/asm/acenv.h
+++ b/arch/x86/include/asm/acenv.h
@@ -10,10 +10,15 @@
 #define _ASM_X86_ACENV_H

 #include <asm/special_insns.h>
+#include <asm/cpu.h>

 /* Asm macros */

-#define ACPI_FLUSH_CPU_CACHE()	wbinvd()
+#define ACPI_FLUSH_CPU_CACHE()					\
+do {								\
+	if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR))	\
+		wbinvd();					\
+} while (0)

 int __acpi_acquire_global_lock(unsigned int *lock);
 int __acpi_release_global_lock(unsigned int *lock);
-- 
2.25.1