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From: Oliver Upton To: Sean Christopherson Subject: Re: [PATCH 0/4] KVM: Fold kvm_arch_sched_in() into kvm_arch_vcpu_load() Message-ID: References: <20240430193157.419425-1-seanjc@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org, Claudio Imbrenda , Janosch Frank , Marc Zyngier , Huacai Chen , Christian Borntraeger , Albert Ou , Bibo Mao , loongarch@lists.linux.dev, Paul Walmsley , kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, Anup Patel , linux-mips@vger.kernel.org, Palmer Dabbelt , kvm-riscv@lists.infradead.org, Paolo Bonzini , Tianrui Zhao , linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Wed, May 01, 2024 at 07:28:21AM -0700, Sean Christopherson wrote: > On Wed, May 01, 2024, Oliver Upton wrote: > > On Tue, Apr 30, 2024 at 12:31:53PM -0700, Sean Christopherson wrote: > > > Drop kvm_arch_sched_in() and instead pass a @sched_in boolean to > > > kvm_arch_vcpu_load(). > > > > > > While fiddling with an idea for optimizing state management on AMD CPUs, > > > I wanted to skip re-saving certain host state when a vCPU is scheduled back > > > in, as the state (theoretically) shouldn't change for the task while it's > > > scheduled out. Actually doing that was annoying and unnecessarily brittle > > > due to having a separate API for the kvm_sched_in() case (the state save > > > needed to be in kvm_arch_vcpu_load() for the common path). > > > > > > E.g. I could have set a "temporary"-ish flag somewhere in kvm_vcpu, but (a) > > > that's gross and (b) it would rely on the arbitrary ordering between > > > sched_in() and vcpu_load() staying the same. > > > > Another option would be to change the rules around kvm_arch_sched_in() > > where the callee is expected to load the vCPU context. > > > > The default implementation could just call kvm_arch_vcpu_load() directly > > and the x86 implementation can order things the way it wants before > > kvm_arch_vcpu_load(). > > > > I say this because ... > > > > > The only real downside I see is that arm64 and riscv end up having to pass > > > "false" for their direct usage of kvm_arch_vcpu_load(), and passing boolean > > > literals isn't ideal. But that can be solved by adding an inner helper that > > > omits the @sched_in param (I almost added a patch to do that, but I couldn't > > > convince myself it was necessary). > > > > Needing to pass @sched_in for other usage of kvm_arch_vcpu_load() hurts > > readability, especially when no other architecture besides x86 cares > > about it. > > Yeah, that bothers me too. > > I tried your suggestion of having x86's kvm_arch_sched_in() do kvm_arch_vcpu_load(), > and even with an added kvm_arch_sched_out() to provide symmetry, the x86 code is > kludgy, and even the common code is a bit confusing as it's not super obvious > that kvm_sched_{in,out}() is really just kvm_arch_vcpu_{load,put}(). > > Staring a bit more at the vCPU flags we have, adding a "bool scheduled_out" isn't > terribly gross if it's done in common code and persists across load() and put(), > i.e. isn't so blatantly a temporary field. And because it's easy, it could be > set with WRITE_ONCE() so that if it can be read cross-task if there's ever a > reason to do so. > > The x86 code ends up being less ugly, and adding future arch/vendor code for > sched_in() *or* sched_out() requires minimal churn, e.g. arch code doesn't need > to override kvm_arch_sched_in(). > > The only weird part is that vcpu->preempted and vcpu->ready have slightly > different behavior, as they are cleared before kvm_arch_vcpu_load(). But the > weirdness is really with those flags no having symmetry, not with scheduled_out > itself. > > Thoughts? Yeah, this seems reasonable. Perhaps scheduled_out could be a nice hint for guardrails / sanity checks in the future. -- Thanks, Oliver