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

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Date: Tue, 19 Mar 2024 14:47:47 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>, "Yamahata, Isaku"
	<isaku.yamahata@...el.com>
CC: "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

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.