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

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Date:   Thu, 23 Jun 2022 21:52:38 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Peter Xu <peterx@...hat.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, Peter Xu wrote:
> 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?

No.

>   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. :)

For the number to really matter, it'd have to be comically large, e.g. 100+.  This
is all on-stack memory, so it's as close to free as can we can get.  Overhead in
terms of (un)marshalling is likely a wash for flags versus bools.  Bools pack in
nicely, so until there are a _lot_ of bools, memory is a non-issue.

That leaves readability, which isn't dependent on the number so much as it is on
the usage, and will be highly subjective based on the final code.

In other words, I'm not dead set against flags, but I would like to see a complete
cleanup before making a decision.  My gut reaction is to use bools, as it makes
consumption cleaner in most cases, e.g.

	if (!(xxx->write_fault || writable))
		return false;

versus

	if (!((xxx->flags & KVM_GTP_WRITE) || writable))
		return false;

but again I'm not going to say never until I actually see the end result.