Re: [PATCH 1/4] PF: Add FAULT_FLAG_RETRY_NOWAIT for guest fault

[Gleb Natapov <gleb@redhat.com>]

On Tue, Jul 09, 2013 at 03:56:44PM +0200, Dominik Dingel wrote:
> In case of a fault retry exit sie64() with gmap_fault indication for the
> running thread set. This makes it possible to handle async page faults
> without the need for mm notifiers.
> 
> Based on a patch from Martin Schwidefsky.
> 
For that we will obviously need Christian and Cornelia ACKs. Or it can
go in via S390 tree.

> Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
> ---
>  arch/s390/include/asm/pgtable.h   |  2 ++
>  arch/s390/include/asm/processor.h |  1 +
>  arch/s390/kvm/kvm-s390.c          | 13 +++++++++++++
>  arch/s390/mm/fault.c              | 26 ++++++++++++++++++++++----
>  4 files changed, 38 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
> index 0ea4e59..4a4cc64 100644
> --- a/arch/s390/include/asm/pgtable.h
> +++ b/arch/s390/include/asm/pgtable.h
> @@ -740,6 +740,7 @@ static inline void pgste_set_pte(pte_t *ptep, pte_t entry)
>   * @table: pointer to the page directory
>   * @asce: address space control element for gmap page table
>   * @crst_list: list of all crst tables used in the guest address space
> + * @pfault_enabled: defines if pfaults are applicable for the guest
>   */
>  struct gmap {
>  	struct list_head list;
> @@ -748,6 +749,7 @@ struct gmap {
>  	unsigned long asce;
>  	void *private;
>  	struct list_head crst_list;
> +	unsigned long pfault_enabled;
>  };
>  
>  /**
> diff --git a/arch/s390/include/asm/processor.h b/arch/s390/include/asm/processor.h
> index 6b49987..4fa96ca 100644
> --- a/arch/s390/include/asm/processor.h
> +++ b/arch/s390/include/asm/processor.h
> @@ -77,6 +77,7 @@ struct thread_struct {
>          unsigned long ksp;              /* kernel stack pointer             */
>  	mm_segment_t mm_segment;
>  	unsigned long gmap_addr;	/* address of last gmap fault. */
> +	unsigned int gmap_pfault;	/* signal of a pending guest pfault */
>  	struct per_regs per_user;	/* User specified PER registers */
>  	struct per_event per_event;	/* Cause of the last PER trap */
>  	unsigned long per_flags;	/* Flags to control debug behavior */
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index ba694d2..702daca 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -682,6 +682,15 @@ static int kvm_s390_handle_requests(struct kvm_vcpu *vcpu)
>  	return 0;
>  }
>  
> +static void kvm_arch_fault_in_sync(struct kvm_vcpu *vcpu)
> +{
> +	hva_t fault = gmap_fault(current->thread.gmap_addr, vcpu->arch.gmap);
> +	struct mm_struct *mm = current->mm;
> +	down_read(&mm->mmap_sem);
> +	get_user_pages(current, mm, fault, 1, 1, 0, NULL, NULL);
> +	up_read(&mm->mmap_sem);
> +}
> +
>  static int __vcpu_run(struct kvm_vcpu *vcpu)
>  {
>  	int rc;
> @@ -715,6 +724,10 @@ static int __vcpu_run(struct kvm_vcpu *vcpu)
>  	if (rc < 0) {
>  		if (kvm_is_ucontrol(vcpu->kvm)) {
>  			rc = SIE_INTERCEPT_UCONTROL;
> +		} else if (current->thread.gmap_pfault) {
> +			kvm_arch_fault_in_sync(vcpu);
> +			current->thread.gmap_pfault = 0;
> +			rc = 0;
>  		} else {
>  			VCPU_EVENT(vcpu, 3, "%s", "fault in sie instruction");
>  			trace_kvm_s390_sie_fault(vcpu);
> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> index 047c3e4..7d4c4b1 100644
> --- a/arch/s390/mm/fault.c
> +++ b/arch/s390/mm/fault.c
> @@ -50,6 +50,7 @@
>  #define VM_FAULT_BADMAP		0x020000
>  #define VM_FAULT_BADACCESS	0x040000
>  #define VM_FAULT_SIGNAL		0x080000
> +#define VM_FAULT_PFAULT		0x100000
>  
>  static unsigned long store_indication __read_mostly;
>  
> @@ -232,6 +233,7 @@ static noinline void do_fault_error(struct pt_regs *regs, int fault)
>  			return;
>  		}
>  	case VM_FAULT_BADCONTEXT:
> +	case VM_FAULT_PFAULT:
>  		do_no_context(regs);
>  		break;
>  	case VM_FAULT_SIGNAL:
> @@ -269,6 +271,9 @@ static noinline void do_fault_error(struct pt_regs *regs, int fault)
>   */
>  static inline int do_exception(struct pt_regs *regs, int access)
>  {
> +#ifdef CONFIG_PGSTE
> +	struct gmap *gmap;
> +#endif
>  	struct task_struct *tsk;
>  	struct mm_struct *mm;
>  	struct vm_area_struct *vma;
> @@ -307,9 +312,10 @@ static inline int do_exception(struct pt_regs *regs, int access)
>  	down_read(&mm->mmap_sem);
>  
>  #ifdef CONFIG_PGSTE
> -	if ((current->flags & PF_VCPU) && S390_lowcore.gmap) {
> -		address = __gmap_fault(address,
> -				     (struct gmap *) S390_lowcore.gmap);
> +	gmap = (struct gmap *)
> +		((current->flags & PF_VCPU) ? S390_lowcore.gmap : 0);
> +	if (gmap) {
> +		address = __gmap_fault(address, gmap);
>  		if (address == -EFAULT) {
>  			fault = VM_FAULT_BADMAP;
>  			goto out_up;
> @@ -318,6 +324,8 @@ static inline int do_exception(struct pt_regs *regs, int access)
>  			fault = VM_FAULT_OOM;
>  			goto out_up;
>  		}
> +		if (test_bit(1, &gmap->pfault_enabled))
> +			flags |= FAULT_FLAG_RETRY_NOWAIT;
>  	}
>  #endif
>  
> @@ -374,9 +382,19 @@ retry:
>  				      regs, address);
>  		}
>  		if (fault & VM_FAULT_RETRY) {
> +#ifdef CONFIG_PGSTE
> +			if (gmap && (flags & FAULT_FLAG_RETRY_NOWAIT)) {
> +				/* FAULT_FLAG_RETRY_NOWAIT has been set,
> +				 * mmap_sem has not been released */
> +				current->thread.gmap_pfault = 1;
> +				fault = VM_FAULT_PFAULT;
> +				goto out_up;
> +			}
> +#endif
>  			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
>  			 * of starvation. */
> -			flags &= ~FAULT_FLAG_ALLOW_RETRY;
> +			flags &= ~(FAULT_FLAG_ALLOW_RETRY |
> +				   FAULT_FLAG_RETRY_NOWAIT);
>  			flags |= FAULT_FLAG_TRIED;
>  			down_read(&mm->mmap_sem);
>  			goto retry;
> -- 
> 1.8.2.2

--
			Gleb.

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