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

Keith Busch
2025-01-13T15:35:51Z

On Thu, Dec 19, 2024 at 09:30:16PM +0100, Paolo Bonzini wrote:
> On Thu, Dec 19, 2024 at 7:09 PM Keith Busch <kbusch@kernel.org> wrote:
> > > Is crosvm trying to do anything but exec? If not, it should probably use the
> > > flag.
> >
> > Good point, and I'm not sure right now. I don't think I know any crosvm
> > developer experts but I'm working on that to get a better explanation of
> > what's happening,
>
> 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, but it did successfully get crosvm
booting again.

---
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;
+
vcpu_load(vcpu);
kvm_sigset_activate(vcpu);
kvm_run->flags = 0;
@@ -12740,11 +12744,6 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
return ret;
}

-int kvm_arch_post_init_vm(struct kvm *kvm)
-{
- return kvm_mmu_post_init_vm(kvm);
-}
-
static void kvm_unload_vcpu_mmu(struct kvm_vcpu *vcpu)
{
vcpu_load(vcpu);
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 401439bb21e3e..a219bd2d8aec8 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1596,7 +1596,6 @@ int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu);
bool kvm_arch_dy_runnable(struct kvm_vcpu *vcpu);
bool kvm_arch_dy_has_pending_interrupt(struct kvm_vcpu *vcpu);
bool kvm_arch_vcpu_preempted_in_kernel(struct kvm_vcpu *vcpu);
-int kvm_arch_post_init_vm(struct kvm *kvm);
void kvm_arch_pre_destroy_vm(struct kvm *kvm);
void kvm_arch_create_vm_debugfs(struct kvm *kvm);

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index de2c11dae2316..adacc6eaa7d9d 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1065,15 +1065,6 @@ static int kvm_create_vm_debugfs(struct kvm *kvm, const char *fdname)
return ret;
}

-/*
- * Called after the VM is otherwise initialized, but just before adding it to
- * the vm_list.
- */
-int __weak kvm_arch_post_init_vm(struct kvm *kvm)
-{
- return 0;
-}
-
/*
* Called just after removing the VM from the vm_list, but before doing any
* other destruction.
@@ -1194,10 +1185,6 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname)
if (r)
goto out_err_no_debugfs;

- r = kvm_arch_post_init_vm(kvm);
- if (r)
- goto out_err;
-
mutex_lock(&kvm_lock);
list_add(&kvm->vm_list, &vm_list);
mutex_unlock(&kvm_lock);
@@ -1207,8 +1194,6 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname)

return kvm;

-out_err:
- kvm_destroy_vm_debugfs(kvm);
out_err_no_debugfs:
kvm_coalesced_mmio_free(kvm);
out_no_coalesced_mmio:
--