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

Sean Christopherson
2024-04-26T15:28:25Z

On Fri, Apr 26, 2024, Rick P Edgecombe wrote:
> On Fri, 2024-04-26 at 08:39 +0100, Fuad Tabba wrote:
> > > 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):

Hmm, yeah, I buy that argument. We could even harded further by poisoning '0'
to force KVM to explicitly. Aha! And maybe use a bitmap?

enum {
BUGGY_KVM_INVALIDATION = 0,
PROCESS_SHARED = BIT(0),
PROCESS_PRIVATE = BIT(1),
PROCESS_PRIVATE_AND_SHARED = PROCESS_SHARED | PROCESS_PRIVATE,
};

> > https://lore.kernel.org/all/ZUO1Giju0GkUdF0o@google.com/
>
> Currently in our internal branch we switched to:
> exclude_private
> exclude_shared
>
> It came together bettter in the code that uses it.

If the choice is between an enum and exclude_*, I would strongly prefer the enum.
Using exclude_* results in inverted polarity for the code that triggers invalidations.

> But I started to wonder if we actually really need exclude_shared. For TDX
> zapping private memory has to be done with more care, because it cannot be re-
> populated without guest coordination. But for shared memory if we are zapping a
> range that includes both private and shared memory, I don't think it should hurt
> to zap the shared memory.

Hell no, I am not risking taking on more baggage in KVM where userspace or some
other subsystem comes to rely on KVM spuriously zapping SPTEs in response to an
unrelated userspace action.