Re: [PATCH 7/8] kvm/arm64: Implement KVM_CAP_MEM_FAULT_NOWAIT for arm64

[Oliver Upton <oliver.upton@linux.dev>]

On Wed, Feb 15, 2023 at 01:16:13AM +0000, Anish Moorthy wrote:
> Just do atomic gfn_to_pfn_memslot when the cap is enabled. Since we
> don't have to deal with async page faults, the implementation is even
> simpler than on x86

All of Sean's suggestions about writing a change description apply here
too.

> Signed-off-by: Anish Moorthy <amoorthy@google.com>
> Acked-by: James Houghton <jthoughton@google.com>
> ---
>  arch/arm64/kvm/arm.c |  1 +
>  arch/arm64/kvm/mmu.c | 14 ++++++++++++--
>  2 files changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 698787ed87e92..31bec7866c346 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -220,6 +220,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  	case KVM_CAP_VCPU_ATTRIBUTES:
>  	case KVM_CAP_PTP_KVM:
>  	case KVM_CAP_ARM_SYSTEM_SUSPEND:
> +	case KVM_CAP_MEM_FAULT_NOWAIT:
>  		r = 1;
>  		break;
>  	case KVM_CAP_SET_GUEST_DEBUG2:
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 01352f5838a00..964af7cd5f1c8 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1206,6 +1206,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  	unsigned long vma_pagesize, fault_granule;
>  	enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R;
>  	struct kvm_pgtable *pgt;
> +	bool mem_fault_nowait;
>  
>  	fault_granule = 1UL << ARM64_HW_PGTABLE_LEVEL_SHIFT(fault_level);
>  	write_fault = kvm_is_write_fault(vcpu);
> @@ -1301,8 +1302,17 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  	 */
>  	smp_rmb();
>  
> -	pfn = __gfn_to_pfn_memslot(memslot, gfn, false, false, NULL,
> -				   write_fault, &writable, NULL);
> +	mem_fault_nowait = memory_faults_enabled(vcpu->kvm);
> +	pfn = __gfn_to_pfn_memslot(
> +		memslot, gfn, mem_fault_nowait, false, NULL,
> +		write_fault, &writable, NULL);
> +
> +	if (mem_fault_nowait && pfn == KVM_PFN_ERR_FAULT) {
> +		vcpu->run->exit_reason = KVM_EXIT_MEMORY_FAULT;
> +		vcpu->run->memory_fault.gpa = gfn << PAGE_SHIFT;
> +		vcpu->run->memory_fault.size = vma_pagesize;
> +		return -EFAULT;

We really don't want to get out to userspace with EFAULT. Instead, we
should get out to userspace with 0 as the return code to indicate a
'normal' / expected exit.

That will require a bit of redefinition on user_mem_abort()'s return
values:

 - < 0, return to userspace with an error
 - 0, return to userspace for a 'normal' exit
 - 1, resume the guest

-- 
Thanks,
Oliver