linux-kernel - Re: [PATCH 2/4] kvm: Merge "atomic" and "write" in __gfn_to_pfn

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Date:   Thu, 23 Jun 2022 17:29:13 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linux MM Mailing List <linux-mm@...ck.org>
Subject: Re: [PATCH 2/4] kvm: Merge "atomic" and "write" in
 __gfn_to_pfn_memslot()

On Thu, Jun 23, 2022 at 08:29:13PM +0000, Sean Christopherson wrote:
> This is what I came up with for splitting @async into a pure input (no_wait) and
> a return value (KVM_PFN_ERR_NEEDS_IO).

The attached patch looks good to me.  It's just that..

[...]

>  kvm_pfn_t __gfn_to_pfn_memslot(const struct kvm_memory_slot *slot, gfn_t gfn,
> -			       bool atomic, bool *async, bool write_fault,
> +			       bool atomic, bool no_wait, bool write_fault,
>  			       bool *writable, hva_t *hva)

.. with this patch on top we'll have 3 booleans already.  With the new one
to add separated as suggested then it'll hit 4.

Let's say one day we'll have that struct, but.. are you sure you think
keeping four booleans around is nicer than having a flag, no matter whether
we'd like to have a struct or not?

  kvm_pfn_t __gfn_to_pfn_memslot(const struct kvm_memory_slot *slot, gfn_t gfn,
			       bool atomic, bool no_wait, bool write_fault,
                               bool interruptible, bool *writable, hva_t *hva);

What if the booleans goes to 5, 6, or more?

/me starts to wonder what'll be the magic number that we'll start to think
a bitmask flag will be more lovely here. :)

-- 
Peter Xu