Re: [PATCH v2 1/4] kvm: x86/pmu: Introduce masked events to the pmu event filter

From: Aaron Lewis <aaronlewis@google.com>
To: Jim Mattson <jmattson@google.com>
Cc: kvm@vger.kernel.org, pbonzini@redhat.com, seanjc@google.com
Subject: Re: [PATCH v2 1/4] kvm: x86/pmu: Introduce masked events to the pmu event filter
Date: Wed, 6 Jul 2022 16:11:29 +0000	[thread overview]
Message-ID: <CAAAPnDHFMdxpsP6TJywQWoMtOudYpc4Z3+pNq7OJA6223L0mcg@mail.gmail.com> (raw)
In-Reply-To: <CALMp9eT3U+kLJTJJ_QP66LQPTQywVTuxucx=7JU74Xb7=xeY0g@mail.gmail.com>

> > +In this mode each event in the events field will be encoded with mask, match,
> > +and invert values in addition to an eventsel.  These encoded events will be
> > +matched against the event the guest is attempting to program to determine
> > +whether the guest should have access to it.  When matching an encoded event
> > +with a guest event these steps are followed:
> > + 1. Match the encoded eventsel to the guest eventsel.
> > + 2. If that matches, match the mask and match values from the encoded event to
> > +    the guest's unit mask (ie: unit_mask & mask == match).
> > + 3. If that matches, the guest is allow to program the event if its an allow
> > +    list or the guest is not allow to program the event if its a deny list.
> > + 4. If the invert value is set in the encoded event, reverse the meaning of #3
> > +    (ie: deny if its an allow list, allow if it's a deny list).
>
> The invert flag introduces some ambiguity. What if a particular event
> matches two of the masked filter entries: one with an invert flag and
> one without?
>

That's a good point!  I think I can deal with that by validating the
events when they are being set to ensure that for a particular event
the invert flags are all set the same way and return EINVAL if they're
not.