Re: [PATCH v19 011/130] KVM: Add new members to struct kvm_gfn_range to operate on

Edgecombe, Rick P
2024-03-19T14:47:56Z

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.