Re: [PATCH v13 12/35] KVM: Prepare for handling only shared mappings in mmu_notifier events

[Fuad Tabba]

On Thu, Nov 2, 2023 at 2:41 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Thu, Nov 02, 2023, Fuad Tabba wrote:
> > Hi,
> >
> > On Fri, Oct 27, 2023 at 7:22 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > >
> > > Add flags to "struct kvm_gfn_range" to let notifier events target only
> > > shared and only private mappings, and write up the existing mmu_notifier
> > > events to be shared-only (private memory is never associated with a
> > > userspace virtual address, i.e. can't be reached via mmu_notifiers).
> > >
> > > Add two flags so that KVM can handle the three possibilities (shared,
> > > private, and shared+private) without needing something like a tri-state
> > > enum.
> > >
> > > Link: https://lore.kernel.org/all/ZJX0hk+KpQP0KUyB@xxxxxxxxxx
> > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > > ---
> > >  include/linux/kvm_host.h | 2 ++
> > >  virt/kvm/kvm_main.c      | 7 +++++++
> > >  2 files changed, 9 insertions(+)
> > >
> > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > > index 96aa930536b1..89c1a991a3b8 100644
> > > --- a/include/linux/kvm_host.h
> > > +++ b/include/linux/kvm_host.h
> > > @@ -263,6 +263,8 @@ struct kvm_gfn_range {
> > >         gfn_t start;
> > >         gfn_t end;
> > >         union kvm_mmu_notifier_arg arg;
> > > +       bool only_private;
> > > +       bool only_shared;
> >
> > If these flags aren't used in this patch series, should this patch be
> > moved to the other series?
>
> If *both* TDX and SNP need this patch, then I think it's probably worth applying
> it now to make their lives easier.  But if only one needs the support, then I
> completely agree this should be punted to whichever series needs it (this also
> came up in v11, but we didn't force the issue).
>
> Mike, Isaku?
>
> > Also, if shared+private is a possibility, doesn't the prefix "only_"
> > confuse things a bit? I.e., what is shared+private, is it when both
> > are 0 or when both are 1? I assume it's the former (both are 0), but
> > it might be clearer.
>
> Heh, I was hoping that "only_private && only_shared" would be obviously nonsensical.
>
> The only alternative I can think would be to add an enum, e.g.
>
>         enum {
>                 PROCESS_PRIVATE_AND_SHARED,
>                 PROCESS_ONLY_PRIVATE,
>                 PROCESS_ONLY_SHARED,
>         };
>
> because every other way of expressing the flags either results in more confusion
> or an unsafe default.  I.e. I want zapping only private or only shared to require
> the caller to explicitly set a non-zero value, which is how I ended up with
> "only_{private,shared}" as opposed to "process_{private,shared}".

I don't have a strong opinion about this. Having an enum looks good to me.

Cheers,
/fuad