Re: [PATCH v2 2/5] x86/idle: Disable IBRS when cpu is offline

From: Waiman Long
Date: Wed Jun 21 2023 - 10:45:53 EST


On 6/21/23 10:36, Peter Zijlstra wrote:
On Wed, Jun 21, 2023 at 09:59:52AM -0400, Waiman Long wrote:
On 6/21/23 03:23, Peter Zijlstra wrote:
On Tue, Jun 20, 2023 at 10:06:22AM -0400, Waiman Long wrote:
Commit bf5835bcdb96 ("intel_idle: Disable IBRS during long idle")
disables IBRS when the CPU enters long idle. However, when a CPU becomes
offline, the IBRS bit is still set when X86_FEATURE_KERNEL_IBRS is
enabled. That will impact the performance of a sibling CPU. Mitigate
this performance impact by clearing all the mitigation bits in SPEC_CTRL
MSR when offline and restoring the value of the MSR when it becomes
online again.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
arch/x86/kernel/smpboot.c | 13 +++++++++++++
1 file changed, 13 insertions(+)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 352f0ce1ece4..5ff82fef413c 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -84,6 +84,7 @@
#include <asm/hw_irq.h>
#include <asm/stackprotector.h>
#include <asm/sev.h>
+#include <asm/nospec-branch.h>
/* representing HT siblings of each logical CPU */
DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_sibling_map);
@@ -1838,12 +1839,24 @@ void __noreturn hlt_play_dead(void)
void native_play_dead(void)
{
+ u64 spec_ctrl = spec_ctrl_current();
+
+ if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS)) {
+ this_cpu_write(x86_spec_ctrl_current, 0);
+ native_wrmsrl(MSR_IA32_SPEC_CTRL, 0);
+ }
+
play_dead_common();
tboot_shutdown(TB_SHUTDOWN_WFS);
mwait_play_dead();
if (cpuidle_play_dead())
hlt_play_dead();
+
+ if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS)) {
+ native_wrmsrl(MSR_IA32_SPEC_CTRL, spec_ctrl);
+ this_cpu_write(x86_spec_ctrl_current, spec_ctrl);
+ }
}
play_dead() is marked __noreturn
There are different versions of play_dead() in the kernel. Some of them are
indeed marked __noreturn like the non-SMP one in arch/x86/kernel/process.c.
The native_play_dead() that I am patching isn't one of those.
mostly by accident I think, hlt_play_dead() is, so I'm thinking
everybody (all compiler and objtool) managed to figure out
native_play_dead() is __noreturn too.

Well, hlt_play_dead() is only called if cpuidle_play_dead() returns an error which is not the typical case. My testing does confirm that this patch is able to keep the IBRS bit off when a CPU is offline via its online sysfs file.

Cheers,
Longman