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

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Date: Tue, 19 Mar 2024 14:50:15 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>,
	"Yamahata, Isaku" <isaku.yamahata@...el.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

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