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From: Oliver Upton To: Sean Christopherson Cc: Marc Zyngier , Tianrui Zhao , Bibo Mao , Huacai Chen , Michael Ellerman , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Paolo Bonzini , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, kvm@vger.kernel.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/6] KVM: Add a flag to track if a loaded vCPU is scheduled out Message-ID: References: <20240522014013.1672962-1-seanjc@google.com> <20240522014013.1672962-2-seanjc@google.com> Precedence: bulk X-Mailing-List: linux-mips@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: <20240522014013.1672962-2-seanjc@google.com> X-Migadu-Flow: FLOW_OUT On Tue, May 21, 2024 at 06:40:08PM -0700, Sean Christopherson wrote: > Add a kvm_vcpu.scheduled_out flag to track if a vCPU is in the process of > being scheduled out (vCPU put path), or if the vCPU is being reloaded > after being scheduled out (vCPU load path). In the short term, this will > allow dropping kvm_arch_sched_in(), as arch code can query scheduled_out > during kvm_arch_vcpu_load(). > > Longer term, scheduled_out opens up other potential optimizations, without > creating subtle/brittle dependencies. E.g. it allows KVM to keep guest > state (that is managed via kvm_arch_vcpu_{load,put}()) loaded across > kvm_sched_{out,in}(), if KVM knows the state isn't accessed by the host > kernel. Forcing arch code to coordinate between kvm_arch_sched_{in,out}() > and kvm_arch_vcpu_{load,put}() is awkward, not reusable, and relies on the > exact ordering of calls into arch code. > > Adding scheduled_out also obviates the need for a kvm_arch_sched_out() > hook, e.g. if arch code needs to do something novel when putting vCPU > state. > > And even if KVM never uses scheduled_out for anything beyond dropping > kvm_arch_sched_in(), just being able to remove all of the arch stubs makes > it worth adding the flag. > > Link: https://lore.kernel.org/all/20240430224431.490139-1-seanjc@google.com > Cc: Oliver Upton > Signed-off-by: Sean Christopherson Reviewed-by: Oliver Upton -- Thanks, Oliver