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Date:   Tue, 5 May 2020 10:42:18 +1000
From:   Gavin Shan <gshan@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, x86@...nel.org,
        kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH RFC 6/6] KVM: x86: Switch KVM guest to using interrupts
 for page ready APF delivery

Hi Vitaly,

On 4/29/20 7:36 PM, Vitaly Kuznetsov wrote:
> KVM now supports using interrupt for type 2 APF event delivery (page ready
> notifications). Switch KVM guests to using it when the feature is present.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>   arch/x86/entry/entry_32.S          |  5 ++++
>   arch/x86/entry/entry_64.S          |  5 ++++
>   arch/x86/include/asm/hardirq.h     |  3 +++
>   arch/x86/include/asm/irq_vectors.h |  6 ++++-
>   arch/x86/include/asm/kvm_para.h    |  6 +++++
>   arch/x86/kernel/irq.c              |  9 +++++++
>   arch/x86/kernel/kvm.c              | 42 ++++++++++++++++++++++++++++++
>   7 files changed, 75 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index b67bae7091d7..d574dadcb2a1 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -1475,6 +1475,11 @@ BUILD_INTERRUPT3(hv_stimer0_callback_vector, HYPERV_STIMER0_VECTOR,
>   
>   #endif /* CONFIG_HYPERV */
>   
> +#ifdef CONFIG_KVM_GUEST
> +BUILD_INTERRUPT3(kvm_async_pf_vector, KVM_ASYNC_PF_VECTOR,
> +		 kvm_async_pf_intr)
> +#endif
> +
>   SYM_CODE_START(page_fault)
>   	ASM_CLAC
>   	pushl	$do_page_fault
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 0e9504fabe52..6f127c1a6547 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -1190,6 +1190,11 @@ apicinterrupt3 HYPERVISOR_CALLBACK_VECTOR \
>   	acrn_hv_callback_vector acrn_hv_vector_handler
>   #endif
>   
> +#ifdef CONFIG_KVM_GUEST
> +apicinterrupt3 KVM_ASYNC_PF_VECTOR \
> +	kvm_async_pf_vector kvm_async_pf_intr
> +#endif
> +
>   idtentry debug			do_debug		has_error_code=0	paranoid=1 shift_ist=IST_INDEX_DB ist_offset=DB_STACK_OFFSET
>   idtentry int3			do_int3			has_error_code=0	create_gap=1
>   idtentry stack_segment		do_stack_segment	has_error_code=1
> diff --git a/arch/x86/include/asm/hardirq.h b/arch/x86/include/asm/hardirq.h
> index 07533795b8d2..be0fbb15ad7f 100644
> --- a/arch/x86/include/asm/hardirq.h
> +++ b/arch/x86/include/asm/hardirq.h
> @@ -44,6 +44,9 @@ typedef struct {
>   	unsigned int irq_hv_reenlightenment_count;
>   	unsigned int hyperv_stimer0_count;
>   #endif
> +#ifdef CONFIG_KVM_GUEST
> +	unsigned int kvm_async_pf_pageready_count;
> +#endif
>   } ____cacheline_aligned irq_cpustat_t;
>   
>   DECLARE_PER_CPU_SHARED_ALIGNED(irq_cpustat_t, irq_stat);
> diff --git a/arch/x86/include/asm/irq_vectors.h b/arch/x86/include/asm/irq_vectors.h
> index 889f8b1b5b7f..8879a9ecd908 100644
> --- a/arch/x86/include/asm/irq_vectors.h
> +++ b/arch/x86/include/asm/irq_vectors.h
> @@ -104,7 +104,11 @@
>   #define HYPERV_STIMER0_VECTOR		0xed
>   #endif
>   
> -#define LOCAL_TIMER_VECTOR		0xec
> +#ifdef CONFIG_KVM_GUEST
> +#define KVM_ASYNC_PF_VECTOR		0xec
> +#endif
> +
> +#define LOCAL_TIMER_VECTOR		0xeb
>   
>   #define NR_VECTORS			 256
>   
> diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
> index 9b4df6eaa11a..fde4f21607f9 100644
> --- a/arch/x86/include/asm/kvm_para.h
> +++ b/arch/x86/include/asm/kvm_para.h
> @@ -4,6 +4,7 @@
>   
>   #include <asm/processor.h>
>   #include <asm/alternative.h>
> +#include <linux/interrupt.h>
>   #include <uapi/asm/kvm_para.h>
>   
>   extern void kvmclock_init(void);
> @@ -93,6 +94,11 @@ void kvm_async_pf_task_wake(u32 token);
>   u32 kvm_read_and_reset_pf_reason(void);
>   extern void kvm_disable_steal_time(void);
>   void do_async_page_fault(struct pt_regs *regs, unsigned long error_code, unsigned long address);
> +extern void kvm_async_pf_vector(void);
> +#ifdef CONFIG_TRACING
> +#define trace_kvm_async_pf_vector kvm_async_pf_vector
> +#endif
> +__visible void __irq_entry kvm_async_pf_intr(struct pt_regs *regs);
>   
>   #ifdef CONFIG_PARAVIRT_SPINLOCKS
>   void __init kvm_spinlock_init(void);
> diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
> index c7965ff429c5..a4c2f25ad74d 100644
> --- a/arch/x86/kernel/irq.c
> +++ b/arch/x86/kernel/irq.c
> @@ -159,6 +159,15 @@ int arch_show_interrupts(struct seq_file *p, int prec)
>   				   irq_stats(j)->hyperv_stimer0_count);
>   		seq_puts(p, "  Hyper-V stimer0 interrupts\n");
>   	}
> +#endif
> +#ifdef CONFIG_KVM_GUEST
> +	if (test_bit(KVM_ASYNC_PF_VECTOR, system_vectors)) {
> +		seq_printf(p, "%*s: ", prec, "APF");
> +		for_each_online_cpu(j)
> +			seq_printf(p, "%10u ",
> +				   irq_stats(j)->kvm_async_pf_pageready_count);
> +		seq_puts(p, "  KVM async PF page ready interrupts\n");
> +	}
>   #endif
>   	seq_printf(p, "%*s: %10u\n", prec, "ERR", atomic_read(&irq_err_count));
>   #if defined(CONFIG_X86_IO_APIC)
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 6efe0410fb72..1c00c7ba01ff 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -259,9 +259,39 @@ do_async_page_fault(struct pt_regs *regs, unsigned long error_code, unsigned lon
>   		rcu_irq_exit();
>   		break;
>   	}
> +
> +	if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF_INT))
> +		wrmsrl(MSR_KVM_ASYNC_PF_ACK, 1);
>   }
>   NOKPROBE_SYMBOL(do_async_page_fault);
>   
> +__visible void __irq_entry kvm_async_pf_intr(struct pt_regs *regs)
> +{
> +	u32 token, reason;
> +
> +	entering_ack_irq();
> +
> +	inc_irq_stat(kvm_async_pf_pageready_count);
> +
> +	if (__this_cpu_read(apf_reason.enabled)) {
> +		reason = __this_cpu_read(apf_reason.reason);
> +		if (reason == KVM_PV_REASON_PAGE_READY) {
> +			token = __this_cpu_read(apf_reason.token);
> +			/*
> +			 * Make sure we read 'token' before we reset
> +			 * 'reason' or it can get lost.
> +			 */
> +			mb();
> +			__this_cpu_write(apf_reason.reason, 0);
> +			kvm_async_pf_task_wake(token);
> +		}
> +	}
> +
> +	wrmsrl(MSR_KVM_ASYNC_PF_ACK, 1);
> +
> +	exiting_irq();
> +}
In theory, it's possible the interrupt happens in the context of the
suspended process. With the call to kvm_async_pf_task_wake(), the
suspended process tries to wake up itself, but it seems it's not
working because of aacedf26fb760 ("sched/core: Optimize try_to_wake_up()
for local wakeups").

It's one of issue I observed when enabling async page fault for arm64,
but not sure it's valid to x86.

Thanks,
Gavin

>   static void __init paravirt_ops_setup(void)
>   {
>   	pv_info.name = "KVM";
> @@ -316,10 +346,17 @@ static void kvm_guest_cpu_init(void)
>   		if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF_VMEXIT))
>   			pa |= KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT;
>   
> +		if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF_INT))
> +			pa |= KVM_ASYNC_PF_DELIVERY_AS_INT;
> +
>   		wrmsrl(MSR_KVM_ASYNC_PF_EN, pa);
>   		__this_cpu_write(apf_reason.enabled, 1);
>   		printk(KERN_INFO"KVM setup async PF for cpu %d\n",
>   		       smp_processor_id());
> +
> +		if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF_INT))
> +			wrmsrl(MSR_KVM_ASYNC_PF2, KVM_ASYNC_PF2_ENABLED |
> +			       KVM_ASYNC_PF_VECTOR);
>   	}
>   
>   	if (kvm_para_has_feature(KVM_FEATURE_PV_EOI)) {
> @@ -649,6 +686,11 @@ static void __init kvm_guest_init(void)
>   	if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
>   		apic_set_eoi_write(kvm_guest_apic_eoi_write);
>   
> +	if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF_INT)) {
> +		pr_info("KVM using interrupt for async PF page-ready\n");
> +		alloc_intr_gate(KVM_ASYNC_PF_VECTOR, kvm_async_pf_vector);
> +	}
> +
>   #ifdef CONFIG_SMP
>   	smp_ops.smp_prepare_cpus = kvm_smp_prepare_cpus;
>   	smp_ops.smp_prepare_boot_cpu = kvm_smp_prepare_boot_cpu;
> 

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