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Date:   Wed, 29 Sep 2021 16:20:40 -0700
From:   Andi Kleen <andi@firstfloor.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Song Liu <songliubraving@fb.com>,
        Like Xu <like.xu.linux@gmail.com>, bpf <bpf@vger.kernel.org>,
        Networking <netdev@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>,
        Andrii Nakryiko <andrii@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        "like.xu@linux.intel.com" <like.xu@linux.intel.com>,
        "Liang, Kan" <kan.liang@intel.com>,
        Andi Kleen <andi@firstfloor.org>
Subject: Re: bpf_get_branch_snapshot on qemu-kvm
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> > How should we confirm this? Can we run some tests for this? Or do we
> > need hardware experts' input for this?
> 
> I'll put it on the list to ask the hardware people when I talk to them
> next. But maybe Kan or Andi know without asking.

On modern CPUs it should be sufficient. In fact the perfmon v4 path that
that was recently deleted did it.

I'm not sure about all old CPUs. There was a corner case on older CPUs
(Haswell and Broadwell) with old Microcode where it was needed when entering virtualization
(but that should be fixed now)


-Andi