Re: [PATCH] KVM: x86/mmu: Add "never" option to allow sticky disabling of nx_huge_pages

[Huang, Kai]

On Fri, 2023-06-02 at 08:09 -0700, Sean Christopherson wrote:
> On Fri, Jun 02, 2023, Robert Hoo wrote:
> > On 6/2/2023 8:58 AM, Sean Christopherson wrote:
> > > @@ -6860,15 +6871,29 @@ static int set_nx_huge_pages(const char *val, const struct kernel_param *kp)
> > >   	bool old_val = nx_huge_pages;
> > >   	bool new_val;
> > > +	if (nx_hugepage_mitigation_hard_disabled)
> > > +		return -EPERM;
> > > +
> > >   	/* In "auto" mode deploy workaround only if CPU has the bug. */
> > > -	if (sysfs_streq(val, "off"))
> > > +	if (sysfs_streq(val, "off")) {
> > >   		new_val = 0;
> > > -	else if (sysfs_streq(val, "force"))
> > > +	} else if (sysfs_streq(val, "force")) {
> > >   		new_val = 1;
> > > -	else if (sysfs_streq(val, "auto"))
> > > +	} else if (sysfs_streq(val, "auto")) {
> > >   		new_val = get_nx_auto_mode();
> > > -	else if (kstrtobool(val, &new_val) < 0)
> > > +	} else if (sysfs_streq(val, "never")) {
> > > +		new_val = 0;
> > > +
> > > +		mutex_lock(&kvm_lock);
> > > +		if (!list_empty(&vm_list)) {
> > > +			mutex_unlock(&kvm_lock);
> > > +			return -EBUSY;
> > > +		}
> > > +		nx_hugepage_mitigation_hard_disabled = true;
> > > +		mutex_unlock(&kvm_lock);
> > > +	} else if (kstrtobool(val, &new_val) < 0) {
> > >   		return -EINVAL;
> > > +	}
> > > 
> > 
> > IIUC, (Initially) "auto_off"/"off" --> create some VM --> turn to "never",
> > the created VMs still have those kthreads, but can never be used, until
> > destroyed with VM.
> 
> Shouldn't be able to happen.  The above rejects "never" if vm_list isn't empty,
> i.e. if there are any VMs, and sets nx_hugepage_mitigation_hard_disabled under
> kvm_lock to ensure it can't race with KVM_CREATE_VM.  I forgot to call this out
> in the changelog though.

Also IIUC once set to "never" userspace will not be able to switch to other
modes, unless userspace reloads KVM.  Worth to call out this too?

Acked-by: Kai Huang <kai.huang@xxxxxxxxx>