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From: Oliver Upton To: James Clark Cc: coresight@lists.linaro.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, broonie@kernel.org, maz@kernel.org, suzuki.poulose@arm.com, acme@kernel.org, James Morse , Zenghui Yu , Catalin Marinas , Will Deacon , Mike Leach , Leo Yan , Alexander Shishkin , Anshuman Khandual , Rob Herring , Miguel Luis , Jintack Lim , Ard Biesheuvel , Mark Rutland , Arnd Bergmann , Vincent Donnefort , Kristina Martsenko , Fuad Tabba , Joey Gouly , Akihiko Odaki , Jing Zhang , linux-kernel@vger.kernel.org Subject: Re: [PATCH v4 2/7] arm64: KVM: Use shared area to pass PMU event state to hypervisor Message-ID: References: <20240104162714.1062610-1-james.clark@arm.com> <20240104162714.1062610-3-james.clark@arm.com> <8a908ee8-620a-d9c2-734b-5a6402950072@arm.com> 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: <8a908ee8-620a-d9c2-734b-5a6402950072@arm.com> X-Migadu-Flow: FLOW_OUT On Mon, Feb 05, 2024 at 12:16:53PM +0000, James Clark wrote: > > This now allows the host to program event counters for a protected > > guest. That _might_ be a useful feature behind some debug option, but is > > most definitely *not* something we want to do for pVMs generally. > > Unless I'm missing something, using PMUs on protected guests was added > by 722625c6f4c5b ("KVM: arm64: Reenable pmu in Protected Mode"). This > change is just a refactor that will allow us to add the same behavior > for a similar feature (tracing) without adding yet another copy of some > state before the guest switch. Ha, I had forgotten about that patch (and I had reviewed it!) My interpretation of the intent for that change was to enable the usage of vPMU for non-protected VMs. The situation has changed since then, as we use the shadow state for vCPUs unconditionally in protected mode as of commit be66e67f1750 ("KVM: arm64: Use the pKVM hyp vCPU structure in handle___kvm_vcpu_run()") Protected mode is well understood at this point to be a WIP feature, and that not all things are expected to work with it. Eventually we will need a way to distinguish between 'normal' VMs and true pVMs (i.e. the VMM selected full isolation) in nVHE, but right now what we have enables testing of some isolation features. > > I'm perfectly happy leaving these sorts of features broken for pKVM and > > using the 'normal' way of getting percpu data to the nVHE hypervisor > > otherwise. > > > > I can do that. But do I also disable PMU at the same time in a new > commit? Now that both PMU and tracing is working maybe it would be a > waste to throw that away and hiding it behind an option is better. Or I > can leave the PMU as it is and just keep tracing disabled in pKVM. > > I don't mind either way, my main goal was to get exclude/include guest > tracing working for normal VMs. For pKVM I don't have a strong opinion. Unless someone has strong opinions about making this work in protected mode, I am happy to see tracing support limited to the 'normal' nVHE configuration. The protected feature as a whole is just baggage until upstream support is completed. -- Thanks, Oliver