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Message-ID: <Z9xT4_fwCgp7VSgC@linux.dev> Date: Thu, 20 Mar 2025 17:44:03 +0000 From: Oliver Upton <oliver.upton@...ux.dev> To: Marc Zyngier <maz@...nel.org> Cc: Akihiko Odaki <akihiko.odaki@...nix.com>, Joey Gouly <joey.gouly@....com>, Suzuki K Poulose <suzuki.poulose@....com>, Zenghui Yu <yuzenghui@...wei.com>, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, Kees Cook <kees@...nel.org>, "Gustavo A. R. Silva" <gustavoars@...nel.org>, linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org, devel@...nix.com Subject: Re: [PATCH RFC] KVM: arm64: PMU: Use multiple host PMUs On Thu, Mar 20, 2025 at 09:19:02AM +0000, Marc Zyngier wrote: > On Wed, 19 Mar 2025 18:51:28 +0000, Oliver Upton <oliver.upton@...ux.dev> wrote: > > I'm at least willing to plug my nose and do the following: > > > > 1) When the VMM does not specify a vPMU type: > > > > - We continue to present the 'default' PMU (including event counters) > > to the VM > > > > - KVM ensures that the fixed CPU cycle counter works on any PMUv3 > > implementation in the system, even if it is different from the > > default > > > > - Otherwise, event counters will only count on the default > > implementation and will not count on different PMUs > > I think this is confusing. The CC is counting, but nothing else, and > people using the cycle counters in conjunction with other events (a > very common use case) will not be able to correlate things correctly. > The current behaviour is, for all its sins, at least consistent. You of course have a good point. What Windows is doing is definitely an outlier. > > > > 2) Implement your suggestion of a UAPI where the VMM can select a PMU > > that only has the CPU cycle counter and works on any PMUv3 > > implementation. > > > > Either way KVM will need to have some special case handling of the fixed > > CPU cycle counter. That'd allow users to actually run Windows *now* and > > provide a clear mechanism for userspace to present a less-broken vPMU if > > it cares. > > Honestly, I don't care about one guest or another. My point is that if > we are changing the behaviour of the PMU to deal with this sort of > things, then it has to be a userspace buy-in. I'm fine with just the user buy-in then. But I still do care about the guest compatibility issue, especially since the end user of all this crap is unlikely to know/care about the fine details of the implementation. So, Akihiko, I would *greatly* appreciate it if you propose a complete solution to the problem, including the KVM and VMM patches to make it all work. Thanks, Oliver
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