Re: [PATCH v3] ACPI/processor_idle: Remove dummy wait if kernel is in guest mode

From: Yin Fengwei
Date: Wed Oct 23 2019 - 21:22:23 EST

On 2019/10/23 äå4:45, David Laight wrote:
From: Yin Fengwei
Sent: 23 October 2019 08:50

In function acpi_idle_do_entry(), an ioport access is used for dummy
wait to guarantee hardware behavior. But it could trigger unnecessary
vmexit if kernel is running as guest in virtualization environtment.

If it's in virtualization environment, the deeper C state enter
operation (inb()) will trap to hyervisor. It's not needed to do
dummy wait after the inb() call. So we remove the dummy io port
access to avoid unnecessary VMexit.

We keep dummy io port access to maintain timing for native environment.

Signed-off-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
v2 -> v3:
- Remove dummy io port access totally for virtualization env.

v1 -> v2:
- Use ndelay instead of dead loop for dummy delay.

drivers/acpi/processor_idle.c | 36 ++++++++++++++++++++++++++++++++---
1 file changed, 33 insertions(+), 3 deletions(-)

diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
index ed56c6d20b08..0c4a97dd6917 100644
--- a/drivers/acpi/processor_idle.c
+++ b/drivers/acpi/processor_idle.c
@@ -58,6 +58,17 @@ struct cpuidle_driver acpi_idle_driver = {
DEFINE_PER_CPU(struct acpi_processor_cx * [CPUIDLE_STATE_MAX], acpi_cstate);

+static void (*dummy_wait)(u64 address);
+static void default_dummy_wait(u64 address)
+ inl(address);
+static void default_noop_wait(u64 address)

Just add:

static void wait_for_freeze(void)
#ifdef CONFIG_X86
/* No delay is needed if we are a guest */
if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
/* Dummy wait op - must do something useless after P_LVL2 read
because chipsets cannot guarantee that STPCLK# signal
gets asserted in time to freeze execution properly. */

and use it to replace the inl().
OK. I was trying to avoid any impact to native case.

+#ifdef CONFIG_X86
+ /* For x86, if we are running in guest, we don't need extra
+ * access ioport as dummy wait.
+ */
+ if (boot_cpu_has(X86_FEATURE_HYPERVISOR)) {
+ pr_err("We are in virtual env");
+ dummy_wait = default_noop_wait;
+ } else {
+ pr_err("We are not in virtual env");
+ }

WTF are the pr_err() for???
Sorry. Didn't remove my debug code...

Yin, Fengwei


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