Re: [PATCH v3 07/22] KVM: Annotate -EFAULTs from kvm_vcpu_write_guest_page()

[Peter Xu <peterx@redhat.com>]

On Wed, Apr 12, 2023 at 09:34:55PM +0000, Anish Moorthy wrote:
> Implement KVM_CAP_MEMORY_FAULT_INFO for efaults from
> kvm_vcpu_write_guest_page()
> 
> Signed-off-by: Anish Moorthy <amoorthy@google.com>
> ---
>  virt/kvm/kvm_main.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 63b4285d858d1..b29a38af543f0 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3119,8 +3119,11 @@ int kvm_vcpu_write_guest_page(struct kvm_vcpu *vcpu, gfn_t gfn,
>  			      const void *data, int offset, int len)
>  {
>  	struct kvm_memory_slot *slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
> +	int ret = __kvm_write_guest_page(vcpu->kvm, slot, gfn, data, offset, len);
>  
> -	return __kvm_write_guest_page(vcpu->kvm, slot, gfn, data, offset, len);
> +	if (ret == -EFAULT)
> +		kvm_populate_efault_info(vcpu, gfn * PAGE_SIZE + offset, len);
> +	return ret;
>  }
>  EXPORT_SYMBOL_GPL(kvm_vcpu_write_guest_page);

Why need to trap this?  Is this -EFAULT part of the "scalable userfault"
plan or not?

My previous memory was one can still leave things like copy_to_user() to go
via the userfaults channels which should work in parallel with the new vcpu
MEMORY_FAULT exit.  But maybe the plan changed?

Thanks,

-- 
Peter Xu