From: Oliver Upton <oliver.upton@linux.dev>
To: Sean Christopherson <seanjc@google.com>
Cc: Marc Zyngier <maz@kernel.org>,
Tianrui Zhao <zhaotianrui@loongson.cn>,
Bibo Mao <maobibo@loongson.cn>,
Huacai Chen <chenhuacai@kernel.org>,
Michael Ellerman <mpe@ellerman.id.au>,
Anup Patel <anup@brainfault.org>,
Paul Walmsley <paul.walmsley@sifive.com>,
Palmer Dabbelt <palmer@dabbelt.com>,
Albert Ou <aou@eecs.berkeley.edu>,
Christian Borntraeger <borntraeger@linux.ibm.com>,
Janosch Frank <frankja@linux.ibm.com>,
Claudio Imbrenda <imbrenda@linux.ibm.com>,
Paolo Bonzini <pbonzini@redhat.com>,
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 0/4] KVM: Fold kvm_arch_sched_in() into kvm_arch_vcpu_load()
Date: Wed, 1 May 2024 00:28:15 +0000 [thread overview]
Message-ID: <ZjGMn5tlq8edKZYv@linux.dev> (raw)
In-Reply-To: <20240430193157.419425-1-seanjc@google.com>
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.
--
Thanks,
Oliver
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next prev parent reply other threads:[~2024-05-01 0:28 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-30 19:31 [PATCH 0/4] KVM: Fold kvm_arch_sched_in() into kvm_arch_vcpu_load() Sean Christopherson
2024-04-30 19:31 ` [PATCH 1/4] KVM: Plumb in a @sched_in flag to kvm_arch_vcpu_load() Sean Christopherson
2024-04-30 19:31 ` [PATCH 2/4] KVM: VMX: Move PLE grow/shrink helpers above vmx_vcpu_load() Sean Christopherson
2024-04-30 19:31 ` [PATCH 3/4] KVM: x86: Fold kvm_arch_sched_in() into kvm_arch_vcpu_load() Sean Christopherson
2024-04-30 19:31 ` [PATCH 4/4] KVM: Delete the now unused kvm_arch_sched_in() Sean Christopherson
2024-05-01 0:28 ` Oliver Upton [this message]
2024-05-01 14:28 ` [PATCH 0/4] KVM: Fold kvm_arch_sched_in() into kvm_arch_vcpu_load() Sean Christopherson
2024-05-01 18:01 ` Oliver Upton
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