linux-kernel - Re: [PATCH] KVM: x86: switch hugepage recovery thread to vhost

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Message-ID: <Z4fnkL5-clssIKc-@kbusch-mbp>
Date: Wed, 15 Jan 2025 09:51:28 -0700
From: Keith Busch <kbusch@...nel.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, michael.christie@...cle.com,
	Tejun Heo <tj@...nel.org>, Luca Boccassi <bluca@...ian.org>
Subject: Re: [PATCH] KVM: x86: switch hugepage recovery thread to vhost_task

On Tue, Jan 14, 2025 at 07:06:20PM -0800, Sean Christopherson wrote:
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index 2401606db2604..422b6b06de4fe 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -7415,6 +7415,8 @@ int kvm_mmu_post_init_vm(struct kvm *kvm)
> > >   {
> > >   	if (nx_hugepage_mitigation_hard_disabled)
> > >   		return 0;
> > > +	if (kvm->arch.nx_huge_page_recovery_thread)
> > > +		return 0;
> 
> ...
> 
> > >   	kvm->arch.nx_huge_page_last = get_jiffies_64();
> > >   	kvm->arch.nx_huge_page_recovery_thread = vhost_task_create(
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index c79a8cc57ba42..263363c46626b 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -11463,6 +11463,10 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
> > >   	struct kvm_run *kvm_run = vcpu->run;
> > >   	int r;
> > > +	r = kvm_mmu_post_init_vm(vcpu->kvm);
> > > +	if (r)
> > > +		return r;
> 
> The only lock held at this point is vcpu->mutex, the obvious choices for guarding
> the per-VM task creation are kvm->lock or kvm->mmu_lock, but we definitely don't
> want to blindly take either lock in KVM_RUN.

Thanks for the feedback. Would this otherwise be okay if I use a
different mechanism to ensure the vhost task creation happens only once
per kvm instance? Or are you suggesting the task creation needs to be
somewhere other than KVM_RUN?

My other initial concern was that this makes kvm_destroy_vm less
symmetrical to kvm_create_vm, but that part looks okay: the vcpu that's
being run holds a reference preventing kvm_destroy_vm from being called.