Re: [WIP Patch v2 12/14] KVM: arm64: Implement KVM_CAP_MEMORY_FAULT_NOWAIT

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

On Wed, Mar 15, 2023 at 02:17:36AM +0000, Anish Moorthy wrote:
> When a memslot has the KVM_MEM_MEMORY_FAULT_EXIT flag set, exit to
> userspace upon encountering a page fault for which the userspace
> page tables do not contain a present mapping.
> 
> 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 3bd732eaf0872..f8337e757c777 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_MEMORY_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 735044859eb25..0d04ffc81f783 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 exit_on_memory_fault = kvm_slot_fault_on_absent_mapping(memslot);
>  
>  	fault_granule = 1UL << ARM64_HW_PGTABLE_LEVEL_SHIFT(fault_level);
>  	write_fault = kvm_is_write_fault(vcpu);
> @@ -1303,8 +1304,17 @@ 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);
> +	pfn = __gfn_to_pfn_memslot(
> +		memslot, gfn, exit_on_memory_fault, false, NULL,
> +		write_fault, &writable, NULL);

As stated before [*], this google3-esque style does not match the kernel style
guide. You may want to check if your work machine is setting up a G3-specific
editor configuration behind your back.

[*] https://lore.kernel.org/kvm/Y+0QRsZ4yWyUdpnc@google.com/

> +	if (exit_on_memory_fault && pfn == KVM_PFN_ERR_FAULT) {

nit: I don't think the local is explicitly necessary. I still find this
readable:

	if (pfn == KVM_PFN_ERR_FAULT && kvm_slot_fault_on_absent_mapping(memslot))

> +		vcpu->run->exit_reason = KVM_EXIT_MEMORY_FAULT;
> +		vcpu->run->memory_fault.flags = 0;
> +		vcpu->run->memory_fault.gpa = gfn << PAGE_SHIFT;
> +		vcpu->run->memory_fault.len = vma_pagesize;
> +		return 0;
> +	}
>  	if (pfn == KVM_PFN_ERR_HWPOISON) {
>  		kvm_send_hwpoison_signal(hva, vma_shift);
>  		return 1;
> -- 
> 2.40.0.rc1.284.g88254d51c5-goog
> 
> 

-- 
Thanks,
Oliver