Re: [PATCH 4/5] KVM: Add hypercall queue for paravirt_ops implementation

[Jeremy Fitzhardinge <jeremy-TSDbQ3PG+2Y@public.gmane.org>]

Anthony Liguori wrote:
> Regards,
>
> Anthony Liguori
> ------------------------------------------------------------------------
>
> Subject: [PATCH] KVM: Add hypercall queue for paravirt_ops implementation
> Author: Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
>
> Implemented a hypercall queue that can be used when paravirt_ops lazy mode
> is enabled.  This patch enables queueing of MMU write operations and CR
> updates.  This results in about a 50% bump in kernbench performance.
>
> Signed-off-by: Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
>
> diff --git a/arch/i386/kernel/kvm.c b/arch/i386/kernel/kvm.c
> index 07ce38e..4b323f1 100644
> --- a/arch/i386/kernel/kvm.c
> +++ b/arch/i386/kernel/kvm.c
> @@ -33,8 +33,10 @@ struct kvm_paravirt_state
>  	unsigned long cached_cr[5];
>  	int cr_valid[5];
>  
> -	struct kvm_vmca *vmca;
> +	enum paravirt_lazy_mode mode;
>  	struct kvm_hypercall_entry *queue;
> +
> +	struct kvm_vmca *vmca;
>  	void (*hypercall)(void);
>  
>  	u64 vmca_gpa;
> @@ -42,17 +44,17 @@ struct kvm_paravirt_state
>  
>  static DEFINE_PER_CPU(struct kvm_paravirt_state *, paravirt_state);
>  
> +static int do_hypercall_batching;
>  static int do_mmu_write;
>  static int do_cr_read_caching;
>  static int do_nop_io_delay;
>  static u64 msr_set_vmca;
>  
> -static long kvm_hypercall(unsigned int nr, unsigned long p1,
> -			  unsigned long p2, unsigned long p3,
> -			  unsigned long p4)
> +static long _kvm_hypercall(struct kvm_paravirt_state *state,
> +			   unsigned int nr, unsigned long p1,
> +			   unsigned long p2, unsigned long p3,
> +			   unsigned long p4)
>  {
> -	struct kvm_paravirt_state *state
> -		= per_cpu(paravirt_state, smp_processor_id());
>  	long ret;
>  
>  	asm volatile("call *(%6) \n\t"
> @@ -69,6 +71,55 @@ static long kvm_hypercall(unsigned int nr, unsigned long p1,
>  	return ret;
>  }
>  
> +static int can_defer_hypercall(struct kvm_paravirt_state *state,
> +			       unsigned int nr)
> +{
> +	if (state->mode == PARAVIRT_LAZY_MMU) {
> +		if (nr == KVM_HYPERCALL_MMU_WRITE)
> +			return 1;
> +	} else if (state->mode == PARAVIRT_LAZY_CPU) {
> +		if (nr == KVM_HYPERCALL_SET_CR)
> +			return 1;
> +	}
> +
> +	return 0;
> +}
> +
> +static void _kvm_hypercall_defer(struct kvm_paravirt_state *state,
> +				 unsigned int nr,
> +				 unsigned long p1, unsigned long p2,
> +				 unsigned long p3, unsigned long p4)
> +{
> +	struct kvm_hypercall_entry *entry;
> +
> +	if (state->vmca->queue_index == state->vmca->max_queue_index)
> +		_kvm_hypercall(state, KVM_HYPERCALL_FLUSH, 0, 0, 0, 0);
> +
> +	/* FIXME: are we preempt safe here? */
>   

BUG_ON(preemptible()) would be a reasonable thing to put here to be sure.

> +	entry = &state->queue[state->vmca->queue_index++];
> +	entry->nr = nr;
> +	entry->p1 = p1;
> +	entry->p2 = p2;
> +	entry->p3 = p3;
> +	entry->p4 = p4;
> +}
> +
> +static long kvm_hypercall(unsigned int nr, unsigned long p1,
> +			  unsigned long p2, unsigned long p3,
> +			  unsigned long p4)
> +{
> +	struct kvm_paravirt_state *state
> +		= per_cpu(paravirt_state, smp_processor_id());
>   

Rather than using this here and passing state around, you could use
either x86_read/write_percpu, or get/put_cpu_var (or __get_vpu_var if
you don't need the preempt-disable).

> +	long ret = 0;
> +
> +	if (can_defer_hypercall(state, nr))
> +		_kvm_hypercall_defer(state, nr, p1, p2, p3, p4);
> +	else
> +		ret = _kvm_hypercall(state, nr, p1, p2, p3, p4);
> +
> +	return ret;
> +}
> +
>  /*
>   * No need for any "IO delay" on KVM
>   */
> @@ -107,7 +158,9 @@ static void kvm_write_cr(int reg, unsigned long value)
>  	state->cr_valid[reg] = 1;
>  	state->cached_cr[reg] = value;
>  
> -	if (reg == 0)
> +	if (state->mode == PARAVIRT_LAZY_CPU)
> +		kvm_hypercall(KVM_HYPERCALL_SET_CR, reg, value, 0, 0);
> +	else if (reg == 0)
>  		native_write_cr0(value);
>  	else if (reg == 3)
>  		native_write_cr3(value);
> @@ -218,6 +271,18 @@ static void kvm_pmd_clear(pmd_t *pmdp)
>  	kvm_mmu_write(pmdp, &pmd, sizeof(pmd));
>  }
>  
> +static void kvm_set_lazy_mode(enum paravirt_lazy_mode mode)
> +{
> +	struct kvm_paravirt_state *state
> +		= per_cpu(paravirt_state, smp_processor_id());
> +
> +	if (mode == PARAVIRT_LAZY_FLUSH || mode == PARAVIRT_LAZY_NONE) {
> +		if (state->vmca->queue_index)
> +			_kvm_hypercall(state, KVM_HYPERCALL_FLUSH, 0, 0, 0, 0);
> +	}
> +	state->mode = mode;
>   

No, you don't want to set state->mode to LAZY_FLUSH (its not a mode,
just a action which overloads the interface).

> +}
> +
>  static void paravirt_ops_setup(void)
>  {
>  	paravirt_ops.name = "KVM";
> @@ -249,6 +314,9 @@ static void paravirt_ops_setup(void)
>  		paravirt_ops.set_pud = kvm_set_pud;
>  	}
>  
> +	if (do_hypercall_batching)
> +		paravirt_ops.set_lazy_mode = kvm_set_lazy_mode;
> +
>  	paravirt_ops.paravirt_enabled = 1;
>  
>  	apply_paravirt(__parainstructions, __parainstructions_end);
> @@ -293,6 +361,9 @@ static int paravirt_initialize(void)
>  	if ((edx & KVM_FEATURE_MMU_WRITE))
>  		do_mmu_write = 1;
>  
> +	if ((edx & KVM_FEATURE_HYPERCALL_BATCHING))
> +		do_hypercall_batching = 1;
> +
>  	on_each_cpu(paravirt_activate, NULL, 0, 1);
>  
>  	return 0;
> @@ -303,6 +374,9 @@ static __init void paravirt_free_state(struct kvm_paravirt_state *state)
>  	if (!state)
>  		return;
>  
> +	if (state->queue)
> +		__free_page(pfn_to_page(__pa(state->queue) >> PAGE_SHIFT));
> +
>  	if (state->hypercall)
>  		__free_page(pfn_to_page(__pa(state->hypercall) >> PAGE_SHIFT));
>  
> @@ -329,8 +403,15 @@ static __init struct kvm_paravirt_state *paravirt_alloc_state(void)
>  	if (!state->hypercall)
>  		goto err;
>  
> +	state->queue = (void *)get_zeroed_page(GFP_KERNEL);
> +	if (!state->queue)
> +		goto err;
> +
>  	state->vmca_gpa = __pa(state->vmca);
>  	state->vmca->hypercall_gpa = __pa(state->hypercall);
> +	state->vmca->queue_gpa = __pa(state->queue);
> +	state->vmca->max_queue_index
> +		= (PAGE_SIZE / sizeof(struct kvm_hypercall_entry));
>  
>  	return state;
>  
> diff --git a/drivers/kvm/kvm.h b/drivers/kvm/kvm.h
> index b08272b..d531899 100644
> --- a/drivers/kvm/kvm.h
> +++ b/drivers/kvm/kvm.h
> @@ -291,6 +291,7 @@ struct kvm_vcpu {
>  	gpa_t para_state_gpa;
>  	struct page *para_state_page;
>  	gpa_t hypercall_gpa;
> +	struct page *queue_page;
>  	unsigned long cr4;
>  	unsigned long cr8;
>  	u64 pdptrs[4]; /* pae */
> diff --git a/drivers/kvm/kvm_main.c b/drivers/kvm/kvm_main.c
> index 4f65729..79a2a64 100644
> --- a/drivers/kvm/kvm_main.c
> +++ b/drivers/kvm/kvm_main.c
> @@ -94,7 +94,8 @@ struct vfsmount *kvmfs_mnt;
>  
>  #define KVM_PARAVIRT_FEATURES \
>  	(KVM_FEATURE_VMCA | KVM_FEATURE_NOP_IO_DELAY | \
> -	 KVM_FEATURE_CR_READ_CACHE | KVM_FEATURE_MMU_WRITE)
> +	 KVM_FEATURE_CR_READ_CACHE | KVM_FEATURE_MMU_WRITE | \
> +	 KVM_FEATURE_HYPERCALL_BATCHING)
>  
>  #define KVM_MSR_SET_VMCA	0x87655678
>  
> @@ -1369,6 +1370,24 @@ static int kvm_hypercall_mmu_write(struct kvm_vcpu *vcpu, gva_t addr,
>  	return 0;
>  }
>  
> +static int kvm_hypercall_set_cr(struct kvm_vcpu *vcpu,
> +				u32 reg, unsigned long value)
> +{
> +	switch (reg) {
> +	case 0:
> +		set_cr0(vcpu, value);
> +		break;
> +	case 3:
> +		set_cr3(vcpu, value);
> +		break;
> +	case 4:
> +		set_cr4(vcpu, value);
> +		break;
> +	}
> +
> +	return 0;
> +}
> +
>  static int dispatch_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
>  			      unsigned long p1, unsigned long p2,
>  			      unsigned long p3, unsigned long p4)
> @@ -1376,10 +1395,36 @@ static int dispatch_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
>  	switch (nr) {
>  	case KVM_HYPERCALL_MMU_WRITE:
>  		return kvm_hypercall_mmu_write(vcpu, p1, p2, p3, p4);
> +	case KVM_HYPERCALL_SET_CR:
> +		return kvm_hypercall_set_cr(vcpu, p1, p2);
>  	}
>  	return -ENOSYS;
>  }
>  
> +static int kvm_hypercall_flush(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_hypercall_entry *queue;
> +	struct kvm_vmca *vmca;
> +	int ret = 0;
> +	int i;
> +
> +	queue = kmap(vcpu->queue_page);
> +	vmca = kmap(vcpu->para_state_page);
>   

kmap_atomic?  Or why not keep them mapped all the time?

    J

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