Date:   Wed, 01 Nov 2017 13:00:00 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     mikelley@...hange.microsoft.com
Cc:     gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        devel@...uxdriverproject.org, olaf@...fle.de, apw@...onical.com,
        jasowang@...hat.com, leann.ogasawara@...onical.com,
        marcelo.cerri@...onical.com, sthemmin@...rosoft.com,
        kys@...rosoft.com, Michael Kelley <mikelley@...rosoft.com>
Subject: Re: [PATCH char-misc 1/1] Drivers: hv: vmbus: Implement Direct Mode for stimer0

mikelley@...hange.microsoft.com writes:

> From: Michael Kelley <mikelley@...rosoft.com>
>
> The 2016 version of Hyper-V offers the option to operate the guest VM
> per-vcpu stimer's in Direct Mode, which means the timer interupts on its
> own vector rather than queueing a VMbus message.  Direct Mode reduces
> timer processing overhead in both the hypervisor and the guest, and
> avoids having timer interrupts pollute the VMbus interrupt stream for
> the synthetic NIC and storage.  This patch enables Direct Mode by
> default on stimer0 (which is the only stimer used in Linux on Hyper-V)
> when running on a version of Hyper-V that supports it, with a hv_vmbus
> module parameter for disabling Direct Mode and reverting to the old
> behavior.
>
> Signed-off-by: Michael Kelley <mikelley@...rosoft.com>
> ---
>  arch/x86/include/asm/mshyperv.h    | 14 ++++++++
>  arch/x86/include/uapi/asm/hyperv.h | 26 ++++++++++++++
>  arch/x86/kernel/cpu/mshyperv.c     | 64 +++++++++++++++++++++++++++++++++-
>  drivers/hv/hv.c                    | 71 ++++++++++++++++++++++++++++++++++++--
>  drivers/hv/hyperv_vmbus.h          |  4 ++-
>  5 files changed, 175 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
> index 740dc97..1bba1d7 100644
> --- a/arch/x86/include/asm/mshyperv.h
> +++ b/arch/x86/include/asm/mshyperv.h
> @@ -4,6 +4,8 @@
>  #include <linux/types.h>
>  #include <linux/atomic.h>
>  #include <linux/nmi.h>
> +#include <linux/irq.h>
> +#include <linux/irqdesc.h>
>  #include <asm/io.h>
>  #include <asm/hyperv.h>
>
> @@ -374,4 +376,16 @@ static inline struct ms_hyperv_tsc_page *hv_get_tsc_page(void)
>  	return NULL;
>  }
>  #endif
> +
> +/* Per architecture routines for stimer0 Direct Mode handling.  On x86/x64
> + * there are no percpu actions to take.
> + */
> +#if IS_ENABLED(CONFIG_HYPERV)
> +static inline void hv_enable_stimer0_percpu_irq(int irq) { }
> +static inline void hv_disable_stimer0_percpu_irq(int irq) { }
> +extern int hv_allocate_stimer0_irq(int *irq, int *vector);
> +extern void hv_deallocate_stimer0_irq(int irq);
> +extern void hv_ack_stimer0_interrupt(struct irq_desc *desc);
> +#endif
> +
>  #endif
> diff --git a/arch/x86/include/uapi/asm/hyperv.h b/arch/x86/include/uapi/asm/hyperv.h
> index f65d125..408cf3e 100644
> --- a/arch/x86/include/uapi/asm/hyperv.h
> +++ b/arch/x86/include/uapi/asm/hyperv.h
> @@ -112,6 +112,22 @@
>  #define HV_X64_GUEST_IDLE_STATE_AVAILABLE		(1 << 5)
>  /* Guest crash data handler available */
>  #define HV_X64_GUEST_CRASH_MSR_AVAILABLE		(1 << 10)
> +/* Debug MSRs available */
> +#define HV_X64_DEBUG_MSR_AVAILABLE			(1 << 11)
> +/* Support for Non-Privileged Instruction Execution Prevention is available */
> +#define HV_X64_NPIEP_AVAILABLE				(1 << 12)
> +/* Support for DisableHypervisor is available */
> +#define HV_X64_DISABLE_HYPERVISOR_AVAILABLE		(1 << 13)
> +/* Extended GVA Ranges for Flush Virtual Address list is available */
> +#define HV_X64_EXTENDED_GVA_RANGE_AVAILABLE		(1 << 14)
> +/* Return Hypercall output via XMM registers is available */
> +#define HV_X64_HYPERCALL_XMM_OUTPUT_AVAILABLE		(1 << 15)
> +/* SINT polling mode available */
> +#define HV_X64_SINT_POLLING_MODE_AVAILABLE		(1 << 17)
> +/* Hypercall MSR lock is available */
> +#define HV_X64_HYPERCALL_MSR_LOCK_AVAILABLE		(1 << 18)
> +/* stimer direct mode is available */
> +#define HV_X64_STIMER_DIRECT_MODE_AVAILABLE		(1 << 19)
>
>  /*
>   * Implementation recommendations. Indicates which behaviors the hypervisor
> @@ -300,6 +316,16 @@ enum HV_GENERIC_SET_FORMAT {
>
>  #define HV_SYNIC_STIMER_COUNT		(4)
>
> +/* Hardware IRQ number to use for stimer0 in Direct Mode.  This IRQ is a fake
> + * because stimer's in Direct Mode simply interrupt on the specified vector,
> + * without using a particular IOAPIC pin. But we use the IRQ allocation
> + * machinery, so we need a hardware IRQ #.  This value is somewhat arbitrary,
> + * but it should not be a legacy IRQ (0 to 15), and should fit within the
> + * single IOAPIC (0 to 23) that Hyper-V provides to a guest VM. So any value
> + * between 16 and 23 should be good.
> + */

(nitpick): all comments in the patch are in 'net' style:

 /* This is a
  * comment.
  */

While in Hyper-V files (and x86 in general) the 'normal' style is used:

 /*
  * This is a
  * comment.
  */

But checkpatch.pl doesn't complain.

> +#define HV_STIMER0_IRQNR		18
> +
>  /* Define synthetic interrupt controller message constants. */
>  #define HV_MESSAGE_SIZE			(256)
>  #define HV_MESSAGE_PAYLOAD_BYTE_COUNT	(240)
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index 236324e8..88dc243 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -19,7 +19,10 @@
>  #include <linux/efi.h>
>  #include <linux/interrupt.h>
>  #include <linux/irq.h>
> +#include <linux/irqdomain.h>
> +#include <linux/irqdesc.h>
>  #include <linux/kexec.h>
> +#include <linux/acpi.h>
>  #include <asm/processor.h>
>  #include <asm/hypervisor.h>
>  #include <asm/hyperv.h>
> @@ -27,6 +30,7 @@
>  #include <asm/desc.h>
>  #include <asm/irq_regs.h>
>  #include <asm/i8259.h>
> +#include <asm/irqdomain.h>
>  #include <asm/apic.h>
>  #include <asm/timer.h>
>  #include <asm/reboot.h>
> @@ -69,6 +73,64 @@ void hv_remove_vmbus_irq(void)
>  EXPORT_SYMBOL_GPL(hv_setup_vmbus_irq);
>  EXPORT_SYMBOL_GPL(hv_remove_vmbus_irq);
>
> +
> +/* Routines to do per-architecture handling of stimer0 when in Direct Mode */
> +
> +void hv_ack_stimer0_interrupt(struct irq_desc *desc)
> +{
> +	ack_APIC_irq();
> +}
> +
> +static void allonline_vector_allocation_domain(int cpu, struct cpumask *retmask,
> +				const struct cpumask *mask)
> +{
> +	cpumask_copy(retmask, cpu_online_mask);
> +}
> +
> +int hv_allocate_stimer0_irq(int *irq, int *vector)
> +{
> +	int		localirq;
> +	int		result;
> +	struct irq_data *irq_data;
> +
> +	/* The normal APIC vector allocation domain allows allocation of vectors
> +	 * only for the calling CPU.  So we change the allocation domain to one
> +	 * that allows vectors to be allocated in all online CPUs.  This
> +	 * change is fine in a Hyper-V VM because VMs don't have the usual
> +	 * complement of interrupting devices.
> +	 */
> +	apic->vector_allocation_domain = allonline_vector_allocation_domain;
> +	localirq = acpi_register_gsi(NULL, HV_STIMER0_IRQNR,
> +				ACPI_LEVEL_SENSITIVE, ACPI_ACTIVE_HIGH);
> +	if (localirq < 0) {
> +		pr_err("Cannot register stimer0 gsi. Error %d", localirq);
> +		return -1;
> +	}
> +
> +	/* We pass in a dummy IRQ handler because architecture independent code
> +	 * will later override the IRQ domain interrupt handler and set it to a
> +	 * Hyper-V specific handler.
> +	 */
> +	result = request_irq(localirq, (irq_handler_t)(-1), 0,
> +					"Hyper-V stimer0", NULL);

I remember K. Y. told me he has patches in his stash to implement
Hyper-V-specific APIC. Is this still in works or there is a reason to
not do it? Just curious.

> +	if (result) {
> +		pr_err("Cannot request stimer0 irq. Error %d", result);
> +		acpi_unregister_gsi(localirq);
> +		return -1;
> +	}
> +	irq_data = irq_domain_get_irq_data(x86_vector_domain, localirq);

In theory irq_domain_get_irq_data() can return NULL.

> +	*vector = irqd_cfg(irq_data)->vector;
> +	*irq = localirq;
> +	return 0;
> +}
> +
> +void hv_deallocate_stimer0_irq(int irq)
> +{
> +	free_irq(irq, NULL);
> +	acpi_unregister_gsi(irq);
> +}
> +
> +
>  void hv_setup_kexec_handler(void (*handler)(void))
>  {
>  	hv_kexec_handler = handler;
> @@ -195,7 +257,7 @@ static void __init ms_hyperv_init_platform(void)
>  		hv_host_info_ecx = cpuid_ecx(HVCPUID_VERSION);
>  		hv_host_info_edx = cpuid_edx(HVCPUID_VERSION);
>
> -		pr_info("Hyper-V Host Build:%d-%d.%d-%d-%d.%d\n",
> +		pr_info("Hyper-V: Host Build %d-%d.%d-%d-%d.%d\n",

While this definitely makes logs nicer uhe change is probably not needed
in this particular patch.

>  			hv_host_info_eax, hv_host_info_ebx >> 16,
>  			hv_host_info_ebx & 0xFFFF, hv_host_info_ecx,
>  			hv_host_info_edx >> 24, hv_host_info_edx & 0xFFFFFF);
> diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
> index fe96aab..68ac474 100644
> --- a/drivers/hv/hv.c
> +++ b/drivers/hv/hv.c
> @@ -27,8 +27,12 @@
>  #include <linux/vmalloc.h>
>  #include <linux/hyperv.h>
>  #include <linux/version.h>
> -#include <linux/interrupt.h>
> +#include <linux/irq.h>
> +#include <linux/irqdesc.h>
> +#include <linux/random.h>
> +#include <linux/kernel_stat.h>
>  #include <linux/clockchips.h>
> +#include <linux/module.h>
>  #include <asm/hyperv.h>
>  #include <asm/mshyperv.h>
>  #include "hyperv_vmbus.h"
> @@ -38,6 +42,21 @@ struct hv_context hv_context = {
>  	.synic_initialized	= false,
>  };
>
> +/* If true, we're using Direct Mode for stimer0, and the timer will do it own
> + * interrupt when it expires. If false, stimer0 is not using Direct Mode and
> + * will send a VMbus message when it expires. We prefer to use Direct Mode,
> + * but not all versions of Hyper-V support Direct Mode.
> + *
> + * While Hyper-V provides a total of four stimer's per CPU, Linux use only
> + * stimer0.
> + */
> +static bool	stimer_direct_mode;
> +static int	stimer0_irq;
> +static int	stimer0_vector;
> +static bool	direct_mode_disable;
> +module_param(direct_mode_disable, bool, 0444);
> +MODULE_PARM_DESC(direct_mode_disable, "Set to Y to disable Direct Mode.");
> +
>  #define HV_TIMER_FREQUENCY (10 * 1000 * 1000) /* 100ns period */
>  #define HV_MAX_MAX_DELTA_TICKS 0xffffffff
>  #define HV_MIN_DELTA_TICKS 1
> @@ -52,7 +71,12 @@ int hv_init(void)
>  	hv_context.cpu_context = alloc_percpu(struct hv_per_cpu_context);
>  	if (!hv_context.cpu_context)
>  		return -ENOMEM;
> +	stimer_direct_mode = (ms_hyperv.misc_features &
> +		HV_X64_STIMER_DIRECT_MODE_AVAILABLE) ? true : false;
>
> +	/* Apply boot command line override to the Direct Mode setting */
> +	if (direct_mode_disable)
> +		stimer_direct_mode = false;

Having both direct_mode_disable and stimer_direct_mode seems redundant.

we may write something like

     if (!direct_mode_disable)
         direct_mode_disable = (ms_hyperv.misc_features & HV_X64_STIMER_DIRECT_MODE_AVAILABLE) ? false : true;

and use it everywhere.

>  	return 0;
>  }
>
> @@ -91,6 +115,23 @@ int hv_post_message(union hv_connection_id connection_id,
>  	return status & 0xFFFF;
>  }
>
> +/* ISR for when stimer0 is operating in Direct Mode.  Direct Mode does
> + * not use VMBus or any VMBus messages, so process here and not in the
> + * VMBus driver code.
> + */
> +
> +static void hv_stimer0_isr(struct irq_desc *desc)
> +{
> +	struct hv_per_cpu_context *hv_cpu;
> +
> +	__this_cpu_inc(*desc->kstat_irqs);
> +	__this_cpu_inc(kstat.irqs_sum);
> +	hv_ack_stimer0_interrupt(desc);
> +	hv_cpu = this_cpu_ptr(hv_context.cpu_context);
> +	hv_cpu->clk_evt->event_handler(hv_cpu->clk_evt);
> +	add_interrupt_randomness(desc->irq_data.irq, 0);

AFAIU implementing Hyper-V specific IRQ chip would allow us to drop this
as we would do this on 'normal' Linux IRQ path.

> +}
> +
>  static int hv_ce_set_next_event(unsigned long delta,
>  				struct clock_event_device *evt)
>  {
> @@ -108,6 +149,8 @@ static int hv_ce_shutdown(struct clock_event_device *evt)
>  {
>  	hv_init_timer(HV_X64_MSR_STIMER0_COUNT, 0);
>  	hv_init_timer_config(HV_X64_MSR_STIMER0_CONFIG, 0);
> +	if (stimer_direct_mode)
> +		hv_disable_stimer0_percpu_irq(stimer0_irq);
>

Both hv_disable_stimer0_percpu_irq() and hv_enable_stimer0_percpu_irq()
are no-ops, right? Why don't we mask/unmask the IRQ? I may have missed
something...

>  	return 0;
>  }
> @@ -116,9 +159,24 @@ static int hv_ce_set_oneshot(struct clock_event_device *evt)
>  {
>  	union hv_timer_config timer_cfg;
>
> +	timer_cfg.as_uint64 = 0; /* Zero everything */
>  	timer_cfg.enable = 1;
>  	timer_cfg.auto_enable = 1;
> -	timer_cfg.sintx = VMBUS_MESSAGE_SINT;
> +	if (stimer_direct_mode) {
> +
> +		/* When it expires, the timer will directly interrupt
> +		 * on the specific hardware vector.
> +		 */
> +		timer_cfg.direct_mode = 1;
> +		timer_cfg.apic_vector = stimer0_vector;
> +		hv_enable_stimer0_percpu_irq(stimer0_irq);
> +	} else {
> +		/* When it expires, the timer will generate a VMbus message,
> +		 * to be handled by the normal VMbus interrupt handler.
> +		 */
> +		timer_cfg.direct_mode = 0;
> +		timer_cfg.sintx = VMBUS_MESSAGE_SINT;
> +	}
>  	hv_init_timer_config(HV_X64_MSR_STIMER0_CONFIG, timer_cfg.as_uint64);
>
>  	return 0;
> @@ -191,6 +249,12 @@ int hv_synic_alloc(void)
>  		INIT_LIST_HEAD(&hv_cpu->chan_list);
>  	}
>
> +	if (stimer_direct_mode) {
> +		if (hv_allocate_stimer0_irq(&stimer0_irq, &stimer0_vector))
> +			goto err;
> +		irq_set_handler(stimer0_irq, hv_stimer0_isr);
> +	}
> +
>  	return 0;
>  err:
>  	return -ENOMEM;
> @@ -292,6 +356,9 @@ void hv_synic_clockevents_cleanup(void)
>  	if (!(ms_hyperv.features & HV_X64_MSR_SYNTIMER_AVAILABLE))
>  		return;
>
> +	if (stimer_direct_mode)
> +		hv_deallocate_stimer0_irq(stimer0_irq);
> +
>  	for_each_present_cpu(cpu) {
>  		struct hv_per_cpu_context *hv_cpu
>  			= per_cpu_ptr(hv_context.cpu_context, cpu);
> diff --git a/drivers/hv/hyperv_vmbus.h b/drivers/hv/hyperv_vmbus.h
> index de6f01d..ee8c89b 100644
> --- a/drivers/hv/hyperv_vmbus.h
> +++ b/drivers/hv/hyperv_vmbus.h
> @@ -55,7 +55,9 @@
>  		u64 periodic:1;
>  		u64 lazy:1;
>  		u64 auto_enable:1;
> -		u64 reserved_z0:12;
> +		u64 apic_vector:8;
> +		u64 direct_mode:1;
> +		u64 reserved_z0:3;
>  		u64 sintx:4;
>  		u64 reserved_z1:44;
>  	};

-- 
  Vitaly

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