Re: [PATCH 04/14] KVM: Play nice with read-only memslots when querying host page size

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

On 08/01/20 21:24, Sean Christopherson wrote:
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 5f7f06824c2b..d9aced677ddd 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1418,15 +1418,23 @@ EXPORT_SYMBOL_GPL(kvm_is_visible_gfn);
>  
>  unsigned long kvm_host_page_size(struct kvm_vcpu *vcpu, gfn_t gfn)
>  {
> +	struct kvm_memory_slot *slot;
>  	struct vm_area_struct *vma;
>  	unsigned long addr, size;
>  
>  	size = PAGE_SIZE;
>  
> -	addr = kvm_vcpu_gfn_to_hva(vcpu, gfn);
> -	if (kvm_is_error_hva(addr))
> +	/*
> +	 * Manually do the equivalent of kvm_vcpu_gfn_to_hva() to avoid the
> +	 * "writable" check in __gfn_to_hva_many(), which will always fail on
> +	 * read-only memslots due to gfn_to_hva() assuming writes.
> +	 */
> +	slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
> +	if (!slot || slot->flags & KVM_MEMSLOT_INVALID)
>  		return PAGE_SIZE;
>  
> +	addr = __gfn_to_hva_memslot(slot, gfn);
> +
>  	down_read(&current->mm->mmap_sem);
>  	vma = find_vma(current->mm, addr);
>  	if (!vma)
> 

Even simpler: use kvm_vcpu_gfn_to_hva_prot

-	addr = kvm_vcpu_gfn_to_hva(vcpu, gfn);
+	addr = kvm_vcpu_gfn_to_hva_prot(vcpu, gfn, NULL);

"You are in a maze of twisty little functions, all alike".

Paolo