linux-kernel - Re: [PATCH v2 2/2] intel_idle: Add a new flag to initialize the AMX state

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Date:   Thu, 10 Mar 2022 19:34:27 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
Subject: Re: [PATCH v2 2/2] intel_idle: Add a new flag to initialize the AMX state

On Wed, Mar 9, 2022 at 11:42 PM Chang S. Bae <chang.seok.bae@...el.com> wrote:
>
> The non-initialized AMX state can be the cause of C-state demotion from C6
> to C1E. This low-power idle state may improve power savings and thus result
> in a higher available turbo frequency budget.
>
> This behavior is implementation-specific. Initialize the state for the C6
> entrance of Sapphire Rapids as needed.
>
> Suggested-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
> Tested-by : Zhang Rui <rui.zhang@...el.com>
> Cc: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
> Cc: linux-pm@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> ---
> Changes from v1:
> * Simplify the code with a new flag (Rui).
> * Rebase on Artem's patches for SPR intel_idle.
> * Massage the changelog.
>
> Dependency on the new C-state table for SPR:
> https://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git/commit/?h=linux-next&id=9edf3c0ffef0ec1bed8300315852b5c6a0997130
> ---
>  drivers/idle/intel_idle.c | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
> index 1c7c25909e54..6ecbeffdf785 100644
> --- a/drivers/idle/intel_idle.c
> +++ b/drivers/idle/intel_idle.c
> @@ -54,6 +54,7 @@
>  #include <asm/intel-family.h>
>  #include <asm/mwait.h>
>  #include <asm/msr.h>
> +#include <asm/fpu/api.h>
>
>  #define INTEL_IDLE_VERSION "0.5.1"
>
> @@ -99,6 +100,11 @@ static unsigned int mwait_substates __initdata;
>   */
>  #define CPUIDLE_FLAG_ALWAYS_ENABLE     BIT(15)
>
> +/*
> + * Initialize large xstate for the C6-state entrance.
> + */
> +#define CPUIDLE_FLAG_INIT_XSTATE       BIT(16)
> +
>  /*
>   * MWAIT takes an 8-bit "hint" in EAX "suggesting"
>   * the C-state (top nibble) and sub-state (bottom nibble)
> @@ -136,6 +142,9 @@ static __cpuidle int intel_idle(struct cpuidle_device *dev,
>         if (state->flags & CPUIDLE_FLAG_IRQ_ENABLE)
>                 local_irq_enable();
>
> +       if (state->flags & CPUIDLE_FLAG_INIT_XSTATE)
> +               fpu_idle_fpregs();
> +
>         mwait_idle_with_hints(eax, ecx);
>
>         return index;
> @@ -156,8 +165,12 @@ static __cpuidle int intel_idle(struct cpuidle_device *dev,
>  static __cpuidle int intel_idle_s2idle(struct cpuidle_device *dev,
>                                        struct cpuidle_driver *drv, int index)
>  {
> -       unsigned long eax = flg2MWAIT(drv->states[index].flags);
>         unsigned long ecx = 1; /* break on interrupt flag */
> +       struct cpuidle_state *state = &drv->states[index];
> +       unsigned long eax = flg2MWAIT(state->flags);
> +
> +       if (state->flags & CPUIDLE_FLAG_INIT_XSTATE)
> +               fpu_idle_fpregs();
>
>         mwait_idle_with_hints(eax, ecx);
>
> @@ -792,7 +805,8 @@ static struct cpuidle_state spr_cstates[] __initdata = {
>         {
>                 .name = "C6",
>                 .desc = "MWAIT 0x20",
> -               .flags = MWAIT2flg(0x20) | CPUIDLE_FLAG_TLB_FLUSHED,
> +               .flags = MWAIT2flg(0x20) | CPUIDLE_FLAG_TLB_FLUSHED | \

Why is the backslash at the end of the line needed?

> +                                          CPUIDLE_FLAG_INIT_XSTATE,
>                 .exit_latency = 290,
>                 .target_residency = 800,
>                 .enter = &intel_idle,
> --