From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4EA8626AC5; Tue, 7 Jul 2026 18:20:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783448408; cv=none; b=hCulSriXt3FgexwqVI9uphryGEol5GMOQRVTZG6NNCBz/ZtxWVf+zNtuj/+5McE8WFcyHoo12RdcgOIspXFp62L5RDjPLv392g36BfxkkEvzXJ/n3178tUiKMsacTmuurtidUVcPkSSSl4p+1TGxwRr5Da4qp6d5UnLgsFUeznQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783448408; c=relaxed/simple; bh=I86E8RIIAR6boEDHK81W0I51tU6/MnrPYSU5+rKgpT0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=CB36AO/ByDNDm2HQevYjQjOXGz6RRwbBeDWMtz6vHgXVTK778sTYFxizk6cjSl/iyngWtTlHw6Lb0cGJqOz/qZKtN+RErFCJnl5Heh2nknRzYQm+sOwwQ4UlSh3UevOYILiZ8dG/cF/2c/+EMbIkeQS4MFwsnSlth5npYaayCps= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ep0suLij; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ep0suLij" Received: by smtp.kernel.org (Postfix) with ESMTPSA id ACF961F000E9; Tue, 7 Jul 2026 18:20:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783448407; bh=AWIzIpH8a00nMIO23YrefbZEhT8NbDGs3/Ve9oenG9g=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=ep0suLijWN3eWlVd2Fpqz+yCycdYo+MNXSN0RmNnb84pJ7Ym/zvlHhZ+yQCzN7Fln mBRJ2yMJDx9RWbXnfb1dBoY+nskN7XRxrU3hfUM99tCczOI2mCyvieaC61VDUbyyml 1z2swx2WCvXQFjqpB3T2YArm4OfIGpw0hV9mr5reikdj4MpRhuK3BSQt/l3/RwWO9V PxLSCgIAFC2hf+yYA+dBuRxho7Qh57l7zrQNFjmhlgUP7aSj597ytZycegiTG+FMuq RS8diYtYN1HGOY9ZestohQsiYB9rBLFD+L8zfbiw6Zsf5p/TJoaARjDEFzGzGrAGL4 UaN10YmI1I4mw== Date: Tue, 7 Jul 2026 11:20:05 -0700 From: Oliver Upton To: Akihiko Odaki Cc: Marc Zyngier , Joey Gouly , Suzuki K Poulose , Zenghui Yu , Catalin Marinas , Will Deacon , Kees Cook , "Gustavo A. R. Silva" , Paolo Bonzini , Jonathan Corbet , Shuah Khan , Shuah Khan , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org, devel@daynix.com, kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH v8 1/7] KVM: arm64: Disallow vPMU when pPMUs do not cover all CPUs Message-ID: References: <20260706-hybrid-v8-0-de459617b59d@rsg.ci.i.u-tokyo.ac.jp> <20260706-hybrid-v8-1-de459617b59d@rsg.ci.i.u-tokyo.ac.jp> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue, Jul 07, 2026 at 08:08:03PM +0900, Akihiko Odaki wrote: > On 2026/07/07 2:04, Oliver Upton wrote: > > Hi, > > > > On Mon, Jul 06, 2026 at 07:03:24PM +0900, Akihiko Odaki wrote: > > > Commit ec3eb9ed6081 ("KVM: arm64: PMU: Disallow vPMU on non-uniform > > > PMUVer") made KVM reject vPMU unless the system-wide PMUVer is usable. > > > That covers systems where PMUv3 is absent or non-uniform, as well as > > > systems where IMPDEF PMUv3 sysreg traps are unavailable. > > > > > > However, KVM can still accept vPMU when all CPUs uniformly trap PMUv3 > > > sysregs, but the pPMUs registered with KVM only cover a subset of > > > possible CPUs. > > > > > > Reject vPMU unless the registered pPMUs cover every possible CPU. > > > This avoids carrying support for partial pPMU coverage into the > > > fixed-counters-only UAPI introduced later in the series. > > > > Doesn't CPU hotplug screw this up? I could online a CPU that doesn't > > have a PMU after creating the VM.> > > I'd rather just change ARM64_WORKAROUND_PMUV3_IMPDEF_TRAPS to become a > > system feature. That way any CPU which breaks the system-wide assumption > > cannot be onlined. > > ARM64_WORKAROUND_PMUV3_IMPDEF_TRAPS only says that IMPDEF PMUv3 sysregs are > trapped. It does not say that KVM has a driver-backed PMU usable for PMUv3 > emulation. This patch checks that extra requirement. > > I re-checked CPU hotplug. Onlining a CPU without a PMU later does not make > an accepted VM unsafe, since the check is against cpu_possible_mask. Sorry, I missed that this was against the possible mask. > The problem is the reverse case on ACPI: this check can disable vPMU when a > possible CPU is offline. DT populates supported_cpus at boot, while ACPI > initially populates it only from online CPUs and grows it as matching CPUs > come online. > > That makes this patch too conservative. In practice, I do not expect systems > to mix CPUs with and without a usable PMU. A better approach is probably to > treat such a host as out of spec and add TAINT_CPU_OUT_OF_SPEC. We already > do that for architectural PMUv3 by detecting mismatches in > ID_AA64DFR0_EL1.PMUVer; we can do the same for > ARM64_WORKAROUND_PMUV3_IMPDEF_TRAPS with non-standard PMUs. The presence of the workaround is, by definition, out of spec. I just never bothered tainting the kernel because these machines are already TAINT_CPU_OUT_OF_SPEC by way of the broken VGIC. Ok, so how about you keep the check that you're doing here and promote IMPDEF_TRAPS to a system-wide feature? That would satisfy the two preconditions we have for PMU emulation, which is system register traps and a backing arm_pmu that understands PMUv3 events. Thanks, Oliver