Re: [PATCH] KVM: x86: switch hugepage recovery thread to vhost_task

[Sean Christopherson <seanjc@google.com>]

On Tue, Jan 14, 2025, Paolo Bonzini wrote:
> On 1/13/25 16:35, Keith Busch wrote:
> > > Ok, I found the code and it doesn't exec (e.g.
> > > https://github.com/google/crosvm/blob/b339d3d7/src/crosvm/sys/linux/jail_warden.rs#L122),
> > > so that's not an option. Well, if I understand correctly from a
> > > cursory look at the code, crosvm is creating a jailed child process
> > > early, and then spawns further jails through it; so it's just this
> > > first process that has to cheat.
> > > 
> > > One possibility on the KVM side is to delay creating the vhost_task
> > > until the first KVM_RUN. I don't like it but...
> > 
> > This option is actually kind of appealing in that we don't need to
> > change any application side to filter out kernel tasks, as well as not
> > having a new kernel dependency to even report these types of tasks as
> > kernel threads.
> > 
> > I gave it a quick try. I'm not very familiar with the code here, so not
> > sure if this is thread safe or not,

It's not.

> > but it did successfully get crosvm booting again.
> 
> That looks good to me too.  Would you like to send it with a commit message
> and SoB?

> > ---
> > 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.