linux-kernel - Re: [PATCH v2 1/6] KVM: Add a flag to track if a loaded vCPU is scheduled out

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Date: Wed, 22 May 2024 08:16:07 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Marc Zyngier <maz@...nel.org>, Tianrui Zhao <zhaotianrui@...ngson.cn>,
	Bibo Mao <maobibo@...ngson.cn>, Huacai Chen <chenhuacai@...nel.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Anup Patel <anup@...infault.org>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Janosch Frank <frankja@...ux.ibm.com>,
	Claudio Imbrenda <imbrenda@...ux.ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	kvm@...r.kernel.org, loongarch@...ts.linux.dev,
	linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/6] KVM: Add a flag to track if a loaded vCPU is
 scheduled 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 <oliver.upton@...ux.dev>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Oliver Upton <oliver.upton@...ux.dev>

-- 
Thanks,
Oliver