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

[Yin Fengwei]

On 2019/10/23 äå5:03, Rafael J. Wysocki wrote:
> On Wed, Oct 23, 2019 at 10:45 AM David Laight <David.Laight@xxxxxxxxxx> 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>
> > > ---
> > > ChangeLog:
> > > 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 = {
> > >   static
> > >   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)
> > > +{
> > > +}
> > > +
> >
> > Overengineered...
> > 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))
> >                  return;
> > #endif
> >
> >          /* 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. */
> >          inl(acpi_gbl_FADT.xpm_timer_block.address);
> > }
> >
> > and use it to replace the inl().
>
> I was about to make a similar comment.
OK. Will send v4 soon.

Regards
Yin, Fengwei

>