x86/CPU/AMD: Don’t force the CPB cap when running under a hypervisor [Linux 5.2]

x86/CPU/AMD: Don’t force the CPB cap when running under a hypervisor [Linux 5.2]

This Linux kernel change "x86/CPU/AMD: Don’t force the CPB cap when running under a hypervisor" is included in the Linux 5.2 release. This change is authored by Frank van der Linden <fllinden [at] amazon.com> on Wed May 22 22:17:45 2019 +0000. The commit for this change in Linux stable tree is 2ac44ab (patch).

x86/CPU/AMD: Don't force the CPB cap when running under a hypervisor

For F17h AMD CPUs, the CPB capability ('Core Performance Boost') 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 <[email protected]>
Acked-by: Borislav Petkov <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Cc: [email protected]
Fixes: 0237199186e7 ("x86/CPU/AMD: Set the CPB bit unconditionally on F17h")
Link: http://lkml.kernel.org/r/[email protected]azon.com
[ Minor edits to the changelog. ]
Signed-off-by: Ingo Molnar <[email protected]>

There are 7 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 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 80a405c..8d4e5042 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -824,8 +824,11 @@ 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);
 }

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