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From: Oliver Upton <oliver.upton@linux.dev>
To: Reiji Watanabe <reijiw@google.com>
Cc: Marc Zyngier <maz@kernel.org>, kvmarm@lists.linux.dev,
	kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Jing Zhang <jingzhangos@google.com>,
	Raghavendra Rao Anata <rananta@google.com>
Subject: Re: [PATCH 1/1] KVM: arm64: PMU: Avoid inappropriate use of host's
 PMUVer
Message-ID: <ZIdzxmgt8257Kv09@linux.dev>
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On Sun, Jun 11, 2023 at 09:01:05AM -0700, Reiji Watanabe wrote:

[...]

> > Suppose KVM is running on a v3p5+ implementation, but userspace has set
> > ID_AA64DFR0_EL1.PMUVer to v3p0. In this case the read of PMCEID1_EL0 on
> > the preceding line would advertise the STALL_SLOT event, and KVM fails
> > to mask it due to the ID register value. The fact we do not support the
> > event is an invariant, in the worst case we wind up clearing a bit
> > that's already 0.
> 
> As far as I checked ArmARM, the STALL_SLOT event can be supported on
> any PMUv3 version (including on v3p0).  Assuming that is true, I don't
> see any reason to not expose the event to the guest in this particular
> example. Or can the STALL_SLOT event only be implemented from certain
> versions of PMUv3 ?

Well, users of the event don't get the full picture w/o PMMIR_EL1.SLOTS,
which is only available on v3p4+. We probably should start exposing the
register + event (separate from this change).

> > This is why I'd suggested just unconditionally clearing the bit. While
> 
> When the hardware supports the STALL_SLOT event (again, I assume any
> PMUv3 version hardware can support the event), and the guest's PMUVer
> is older than v3p4, what is the reason why we want to clear the bit ?

What's the value of the event w/o PMMIR_EL1? I agree there's no
fundamental issue with letting it past, but I'd rather we start
exposing the feature when we provide all the necessary detail.

--
Thanks,
Oliver