linux-kernel - [PATCH] x86/CPU/AMD: don't force the CPB cap when running under a hypervisor

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Message-ID: <20190522221745.GA15789@dev-dsk-fllinden-2c-c1893d73.us-west-2.amazon.com>
Date:   Wed, 22 May 2019 22:17:45 +0000
From:   Frank van der Linden <fllinden@...zon.com>
To:     <x86@...nel.org>
CC:     <linux-kernel@...r.kernel.org>, <bp@...en8.de>,
        <jiaxun.yang@...goat.com>
Subject: [PATCH] x86/CPU/AMD: don't force the CPB cap when running under a
 hypervisor

For F17h AMD CPUs, the CPB capability is forcibly set, because some
versions of that chip incorrectly report that they do not have it.

However, a hypervisor may filter out the CPB capability, for good
reasons. For example, KVM currently does not emulate setting the CPB
bit in MSR_K7_HWCR, and unchecked MSR access errors will be thrown
when trying to set it as a guest:

	unchecked MSR access error: WRMSR to 0xc0010015 (tried to write
        0x0000000001000011) at rIP: 0xffffffff890638f4
        (native_write_msr+0x4/0x20)

	Call Trace:
	boost_set_msr+0x50/0x80 [acpi_cpufreq]
	cpuhp_invoke_callback+0x86/0x560
	sort_range+0x20/0x20
	cpuhp_thread_fun+0xb0/0x110
	smpboot_thread_fn+0xef/0x160
	kthread+0x113/0x130
	kthread_create_worker_on_cpu+0x70/0x70
	ret_from_fork+0x35/0x40

To avoid this issue, don't forcibly set the CPB capability for a CPU
when running under a hypervisor.

Signed-off-by: Frank van der Linden <fllinden@...zon.com>
Fixes: 0237199186e7 ("x86/CPU/AMD: Set the CPB bit unconditionally on F17h")
---
 arch/x86/kernel/cpu/amd.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index fb6a64bd765f..ee4d79fa1b19 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -823,8 +823,11 @@ static void init_amd_zn(struct cpuinfo_x86 *c)
 {
 	set_cpu_cap(c, X86_FEATURE_ZEN);
 
-	/* Fix erratum 1076: CPB feature bit not being set in CPUID. */
-	if (!cpu_has(c, X86_FEATURE_CPB))
+	/*
+	 * Fix erratum 1076: CPB feature bit not being set in CPUID.
+	 * Always set it, except when running under a hypervisor.
+	 */
+	if (!cpu_has(c, X86_FEATURE_HYPERVISOR) && !cpu_has(c, X86_FEATURE_CPB))
 		set_cpu_cap(c, X86_FEATURE_CPB);
 }
 
-- 
2.17.2