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Message-ID: <CAJZ5v0jhK51+pkf=Amr=qXWzK3e1xC_tdt0iqQXxVfeE4pcFJQ@mail.gmail.com>
Date: Tue, 10 Dec 2024 20:51:44 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Patryk Wlazlyn <patryk.wlazlyn@...ux.intel.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
rafael.j.wysocki@...el.com, peterz@...radead.org, dave.hansen@...ux.intel.com,
gautham.shenoy@....com, tglx@...utronix.de, len.brown@...el.com,
artem.bityutskiy@...ux.intel.com
Subject: Re: [PATCH v7 1/4] x86/smp: Allow calling mwait_play_dead with an
arbitrary hint
First, I would change the subject to something like "x86/smp: Add hint
parameter to mwait_play_dead()"
On Fri, Nov 29, 2024 at 7:22 PM Patryk Wlazlyn
<patryk.wlazlyn@...ux.intel.com> wrote:
>
> Introduce a helper function to allow offlined CPUs to enter FFh idle
> states with a specific MWAIT hint. The new helper will be used in
> subsequent patches by the acpi_idle and intel_idle drivers.
And the above would become
"Change mwait_play_dead() into a helper function allowing CPUs going
offline to enter idle states via MWAIT with a specific hint passed to
it as an argument.
Add mwait_play_dead_cpuid_hint() as a wrapper around mwait_play_dead()
implementing the existing behavior of the code.
Subsequently, the new helper will also be used by the acpi_idle and
intel_idle drivers in idle-state-specific :enter_dead() callbacks."
> No functional change intended.
>
> Signed-off-by: Patryk Wlazlyn <patryk.wlazlyn@...ux.intel.com>
> ---
> arch/x86/include/asm/smp.h | 3 ++
> arch/x86/kernel/smpboot.c | 90 ++++++++++++++++++++------------------
> 2 files changed, 51 insertions(+), 42 deletions(-)
>
> diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
> index ca073f40698f..dfd09a1e09bf 100644
> --- a/arch/x86/include/asm/smp.h
> +++ b/arch/x86/include/asm/smp.h
> @@ -114,6 +114,7 @@ void wbinvd_on_cpu(int cpu);
> int wbinvd_on_all_cpus(void);
>
> void smp_kick_mwait_play_dead(void);
> +void mwait_play_dead(unsigned int hint);
>
> void native_smp_send_reschedule(int cpu);
> void native_send_call_func_ipi(const struct cpumask *mask);
> @@ -164,6 +165,8 @@ static inline struct cpumask *cpu_llc_shared_mask(int cpu)
> {
> return (struct cpumask *)cpumask_of(0);
> }
> +
> +static inline void mwait_play_dead(unsigned int eax_hint) { }
> #endif /* CONFIG_SMP */
>
> #ifdef CONFIG_DEBUG_NMI_SELFTEST
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index b5a8f0891135..8a3545c2cae9 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1272,13 +1272,57 @@ void play_dead_common(void)
> local_irq_disable();
> }
>
> +void __noreturn mwait_play_dead(unsigned int eax_hint)
> +{
> + struct mwait_cpu_dead *md = this_cpu_ptr(&mwait_cpu_dead);
> +
> + /* Set up state for the kexec() hack below */
> + md->status = CPUDEAD_MWAIT_WAIT;
> + md->control = CPUDEAD_MWAIT_WAIT;
> +
> + wbinvd();
> +
> + while (1) {
> + /*
> + * The CLFLUSH is a workaround for erratum AAI65 for
> + * the Xeon 7400 series. It's not clear it is actually
> + * needed, but it should be harmless in either case.
> + * The WBINVD is insufficient due to the spurious-wakeup
> + * case where we return around the loop.
> + */
> + mb();
> + clflush(md);
> + mb();
> + __monitor(md, 0, 0);
> + mb();
> + __mwait(eax_hint, 0);
> +
> + if (READ_ONCE(md->control) == CPUDEAD_MWAIT_KEXEC_HLT) {
> + /*
> + * Kexec is about to happen. Don't go back into mwait() as
> + * the kexec kernel might overwrite text and data including
> + * page tables and stack. So mwait() would resume when the
> + * monitor cache line is written to and then the CPU goes
> + * south due to overwritten text, page tables and stack.
> + *
> + * Note: This does _NOT_ protect against a stray MCE, NMI,
> + * SMI. They will resume execution at the instruction
> + * following the HLT instruction and run into the problem
> + * which this is trying to prevent.
> + */
> + WRITE_ONCE(md->status, CPUDEAD_MWAIT_KEXEC_HLT);
> + while(1)
> + native_halt();
> + }
> + }
> +}
> +
> /*
> * We need to flush the caches before going to sleep, lest we have
> * dirty data in our caches when we come back up.
> */
> -static inline void mwait_play_dead(void)
> +static inline void mwait_play_dead_cpuid_hint(void)
> {
> - struct mwait_cpu_dead *md = this_cpu_ptr(&mwait_cpu_dead);
> unsigned int eax, ebx, ecx, edx;
> unsigned int highest_cstate = 0;
> unsigned int highest_subcstate = 0;
> @@ -1316,45 +1360,7 @@ static inline void mwait_play_dead(void)
> (highest_subcstate - 1);
> }
>
> - /* Set up state for the kexec() hack below */
> - md->status = CPUDEAD_MWAIT_WAIT;
> - md->control = CPUDEAD_MWAIT_WAIT;
> -
> - wbinvd();
> -
> - while (1) {
> - /*
> - * The CLFLUSH is a workaround for erratum AAI65 for
> - * the Xeon 7400 series. It's not clear it is actually
> - * needed, but it should be harmless in either case.
> - * The WBINVD is insufficient due to the spurious-wakeup
> - * case where we return around the loop.
> - */
> - mb();
> - clflush(md);
> - mb();
> - __monitor(md, 0, 0);
> - mb();
> - __mwait(eax, 0);
> -
> - if (READ_ONCE(md->control) == CPUDEAD_MWAIT_KEXEC_HLT) {
> - /*
> - * Kexec is about to happen. Don't go back into mwait() as
> - * the kexec kernel might overwrite text and data including
> - * page tables and stack. So mwait() would resume when the
> - * monitor cache line is written to and then the CPU goes
> - * south due to overwritten text, page tables and stack.
> - *
> - * Note: This does _NOT_ protect against a stray MCE, NMI,
> - * SMI. They will resume execution at the instruction
> - * following the HLT instruction and run into the problem
> - * which this is trying to prevent.
> - */
> - WRITE_ONCE(md->status, CPUDEAD_MWAIT_KEXEC_HLT);
> - while(1)
> - native_halt();
> - }
> - }
> + mwait_play_dead(eax);
> }
>
> /*
> @@ -1407,7 +1413,7 @@ void native_play_dead(void)
> play_dead_common();
> tboot_shutdown(TB_SHUTDOWN_WFS);
>
> - mwait_play_dead();
> + mwait_play_dead_cpuid_hint();
> if (cpuidle_play_dead())
> hlt_play_dead();
> }
> --
And honestly I'm wondering why adding a parameter to mwait_play_dead()
is better than introducing mwait_play_dead_with_hint(), in analogy
with the existing mwait_idle_with_hints()?
The latter option would allow you to avoid introducing a function that
is deleted in the same patch series (in patch 4).
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