Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:4 as permitted sender) client-ip=2620:137:e000::3:4;
MIME-Version: 1.0
References: <20231031092921.2885109-1-dapeng1.mi@linux.intel.com>
 <20231031092921.2885109-5-dapeng1.mi@linux.intel.com> <CALMp9eQ4Xj5D-kgqVMKUNmdF37rLcMRXyDYdQU339sRCKZ7d9A@mail.gmail.com>
 <28796dd3-ac4e-4a38-b9e1-f79533b2a798@linux.intel.com>
In-Reply-To: <28796dd3-ac4e-4a38-b9e1-f79533b2a798@linux.intel.com>
From:   Jim Mattson <jmattson@google.com>
Date:   Tue, 31 Oct 2023 19:47:50 -0700
Message-ID: <CALMp9eRH5pttOA5BApdVeSbbkOU-kWcOWAoGMfK-9f=cy2Jf0g@mail.gmail.com>
Subject: Re: [kvm-unit-tests Patch v2 4/5] x86: pmu: Support validation for
 Intel PMU fixed counter 3
To:     "Mi, Dapeng" <dapeng1.mi@linux.intel.com>
Cc:     Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Zhenyu Wang <zhenyuw@linux.intel.com>,
        Zhang Xiong <xiong.y.zhang@intel.com>,
        Mingwei Zhang <mizhang@google.com>,
        Like Xu <like.xu.linux@gmail.com>,
        Dapeng Mi <dapeng1.mi@intel.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Tue, Oct 31, 2023 at 7:33=E2=80=AFPM Mi, Dapeng <dapeng1.mi@linux.intel.=
com> wrote:
>
>
> On 11/1/2023 2:47 AM, Jim Mattson wrote:
> > On Tue, Oct 31, 2023 at 2:22=E2=80=AFAM Dapeng Mi <dapeng1.mi@linux.int=
el.com> wrote:
> >> Intel CPUs, like Sapphire Rapids, introduces a new fixed counter
> >> (fixed counter 3) to counter/sample topdown.slots event, but current
> >> code still doesn't cover this new fixed counter.
> >>
> >> So this patch adds code to validate this new fixed counter can count
> >> slots event correctly.
> > I'm not convinced that this actually validates anything.
> >
> > Suppose, for example, that KVM used fixed counter 1 when the guest
> > asked for fixed counter 3. Wouldn't this test still pass?
>
>
> Per my understanding, as long as the KVM returns a valid count in the
> reasonable count range, we can think KVM works correctly. We don't need
> to entangle on how KVM really uses the HW, it could be impossible and
> unnecessary.

Now, I see how the Pentium FDIV bug escaped notice. Hey, the numbers
are in a reasonable range. What's everyone upset about?

> Yeah, currently the predefined valid count range may be some kind of
> loose since I want to cover as much as hardwares and avoid to cause
> regression. Especially after introducing the random jump and clflush
> instructions, the cycles and slots become much more hard to predict.
> Maybe we can have a comparable restricted count range in the initial
> change, and we can loosen the restriction then if we encounter a failure
> on some specific hardware. do you think it's better? Thanks.

I think the test is essentially useless, and should probably just be
deleted, so that it doesn't give a false sense of confidence.