Subject: Re: [PATCH v7 2/8] KVM: Introduce __kvm_follow_pfn function

From: Sean Christopherson <[email-masked]>
Date: Fri, 4 Aug 2023 15:13:28 -0700

On Tue, Jul 04, 2023, David Stevens wrote:
> From: David Stevens <[email-masked]>
> 
> Introduce __kvm_follow_pfn, which will replace __gfn_to_pfn_memslot.
> __kvm_follow_pfn refactors the old API's arguments into a struct and,
> where possible, combines the boolean arguments into a single flags
> argument.
> 
> Signed-off-by: David Stevens <[email-masked]>
> ---
>  include/linux/kvm_host.h |  16 ++++
>  virt/kvm/kvm_main.c      | 171 ++++++++++++++++++++++-----------------
>  virt/kvm/kvm_mm.h        |   3 +-
>  virt/kvm/pfncache.c      |   8 +-
>  4 files changed, 122 insertions(+), 76 deletions(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 9d3ac7720da9..ef2763c2b12e 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -97,6 +97,7 @@
>  #define KVM_PFN_ERR_HWPOISON	(KVM_PFN_ERR_MASK + 1)
>  #define KVM_PFN_ERR_RO_FAULT	(KVM_PFN_ERR_MASK + 2)
>  #define KVM_PFN_ERR_SIGPENDING	(KVM_PFN_ERR_MASK + 3)
> +#define KVM_PFN_ERR_NEEDS_IO	(KVM_PFN_ERR_MASK + 4)

Hmm, ideally KVM_PFN_ERR_NEEDS_IO would be introduced in a separate prep patch,
e.g. by changing "bool *async" to "bool no_wait".  At a glance, I can't tell if
that's feasible though, so consider it more of a "wish" than a request.

> @@ -2572,23 +2561,23 @@ static int kvm_try_get_pfn(kvm_pfn_t pfn)
>  	return get_page_unless_zero(page);
>  }
>  
> -static int hva_to_pfn_remapped(struct vm_area_struct *vma,
> -			       unsigned long addr, bool write_fault,
> -			       bool *writable, kvm_pfn_t *p_pfn)
> +static int hva_to_pfn_remapped(struct vm_area_struct *vma, struct kvm_follow_pfn *foll,
> +			       kvm_pfn_t *p_pfn)

Please wrap.  KVM still honors the 80 char soft limit unless there's a reason not
to, and in this case it's already wrapping

static int hva_to_pfn_remapped(struct vm_area_struct *vma,
			       struct kvm_follow_pfn *foll, kvm_pfn_t *p_pfn)

> @@ -2606,8 +2595,7 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
>  		goto out;
>  	}
>  
> -	if (writable)
> -		*writable = pte_write(*ptep);
> +	foll->writable = pte_write(*ptep) && foll->allow_write_mapping;

Similar to feedback in my other response, don't condition this on try_map_writable,
i.e. just do:

	foll->writable = pte_write(...);

>  	pfn = pte_pfn(*ptep);
>  
>  	/*
> @@ -2652,24 +2640,22 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
>   * 2): @write_fault = false && @writable, @writable will tell the caller
>   *     whether the mapping is writable.
>   */
> -kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool interruptible,
> -		     bool *async, bool write_fault, bool *writable)
> +kvm_pfn_t hva_to_pfn(struct kvm_follow_pfn *foll)
>  {
>  	struct vm_area_struct *vma;
>  	kvm_pfn_t pfn;
>  	int npages, r;
>  
>  	/* we can do it either atomically or asynchronously, not both */
> -	BUG_ON(atomic && async);
> +	BUG_ON(foll->atomic && (foll->flags & FOLL_NOWAIT));
>  
> -	if (hva_to_pfn_fast(addr, write_fault, writable, &pfn))
> +	if (hva_to_pfn_fast(foll, &pfn))
>  		return pfn;
>  
> -	if (atomic)
> +	if (foll->atomic)
>  		return KVM_PFN_ERR_FAULT;
>  
> -	npages = hva_to_pfn_slow(addr, async, write_fault, interruptible,
> -				 writable, &pfn);
> +	npages = hva_to_pfn_slow(foll, &pfn);
>  	if (npages == 1)
>  		return pfn;
>  	if (npages == -EINTR)
> @@ -2677,83 +2663,122 @@ kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool interruptible,
>  
>  	mmap_read_lock(current->mm);
>  	if (npages == -EHWPOISON ||
> -	      (!async && check_user_page_hwpoison(addr))) {
> +	      (!(foll->flags & FOLL_NOWAIT) && check_user_page_hwpoison(foll->hva))) {

Opportunistically align the indentation, as an added bonus that makes the line
length a few chars shorter, i.e.

	if (npages == -EHWPOISON ||
	    (!(foll->flags & FOLL_NOWAIT) && check_user_page_hwpoison(foll->hva))) {
		pfn = KVM_PFN_ERR_HWPOISON;
		goto exit;
	}