linux-kernel - Re: [PATCH v19 011/130] KVM: Add new members to struct kvm_gfn

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Date: Fri, 26 Apr 2024 08:39:26 +0100
From: Fuad Tabba <tabba@...gle.com>
To: Isaku Yamahata <isaku.yamahata@...el.com>
Cc: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, 
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>, "Zhang, Tina" <tina.zhang@...el.com>, 
	"isaku.yamahata@...ux.intel.com" <isaku.yamahata@...ux.intel.com>, "seanjc@...gle.com" <seanjc@...gle.com>, 
	"Huang, Kai" <kai.huang@...el.com>, "sagis@...gle.com" <sagis@...gle.com>, "Chen, Bo2" <chen.bo@...el.com>, 
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, "Aktas, Erdem" <erdemaktas@...gle.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"Yuan, Hang" <hang.yuan@...el.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v19 011/130] KVM: Add new members to struct kvm_gfn_range
 to operate on

Hi,

On Tue, Mar 19, 2024 at 9:50 PM Isaku Yamahata <isaku.yamahata@...el.com> wrote:
>
> On Tue, Mar 19, 2024 at 02:47:47PM +0000,
> "Edgecombe, Rick P" <rick.p.edgecombe@...el.com> wrote:
>
> > On Mon, 2024-03-18 at 19:50 -0700, Rick Edgecombe wrote:
> > > On Wed, 2024-03-13 at 10:14 -0700, Isaku Yamahata wrote:
> > > > > IMO, an enum will be clearer than the two flags.
> > > > >
> > > > >     enum {
> > > > >         PROCESS_PRIVATE_AND_SHARED,
> > > > >         PROCESS_ONLY_PRIVATE,
> > > > >         PROCESS_ONLY_SHARED,
> > > > >     };
> > > >
> > > > The code will be ugly like
> > > > "if (== PRIVATE || == PRIVATE_AND_SHARED)" or
> > > > "if (== SHARED || == PRIVATE_AND_SHARED)"
> > > >
> > > > two boolean (or two flags) is less error-prone.
> > >
> > > Yes the enum would be awkward to handle. But I also thought the way
> > > this is specified in struct kvm_gfn_range is a little strange.
> > >
> > > It is ambiguous what it should mean if you set:
> > >  .only_private=true;
> > >  .only_shared=true;
> > > ...as happens later in the series (although it may be a mistake).
> > >
> > > Reading the original conversation, it seems Sean suggested this
> > > specifically. But it wasn't clear to me from the discussion what the
> > > intention of the "only" semantics was. Like why not?
> > >  bool private;
> > >  bool shared;
> >
> > I see Binbin brought up this point on v18 as well:
> > https://lore.kernel.org/kvm/6220164a-aa1d-43d2-b918-6a6eaad769fb@linux.intel.com/#t
> >
> > and helpfully dug up some other discussion with Sean where he agreed
> > the "_only" is confusing and proposed the the enum:
> > https://lore.kernel.org/kvm/ZUO1Giju0GkUdF0o@google.com/
> >
> > He wanted the default value (in the case the caller forgets to set
> > them), to be to include both private and shared. I think the enum has
> > the issues that Isaku mentioned. What about?
> >
> >  bool exclude_private;
> >  bool exclude_shared;
> >
> > It will become onerous if more types of aliases grow, but it clearer
> > semantically and has the safe default behavior.
>
> I'm fine with those names. Anyway, I'm fine with wither way, two bools or enum.

I don't have a strong opinion, but I'd brought it up in a previous
patch series. I think that having two bools to encode three states is
less intuitive and potentially more bug prone, more so than the naming
itself (i.e., _only):
https://lore.kernel.org/all/ZUO1Giju0GkUdF0o@google.com/

Cheers,
/fuad

> --
> Isaku Yamahata <isaku.yamahata@...el.com>
>