Re: [PATCH v5 4/4] KVM: x86: Don't disable IRQs when unregistering user-return notifier

Huang, Kai
2025-11-04T10:34:29Z

On Thu, 2025-10-30 at 12:15 -0700, Sean Christopherson wrote:
> From: Hou Wenlong <houwenlong.hwl@antgroup.com>
>
> Remove the code to disable IRQs when unregistering KVM's user-return
> notifier now that KVM doesn't invoke kvm_on_user_return() when disabling
> virtualization via IPI function call, i.e. now that there's no need to
> guard against re-entrancy via IPI callback.
>
> Note, disabling IRQs has largely been unnecessary since commit
> a377ac1cd9d7b ("x86/entry: Move user return notifier out of loop") moved
> fire_user_return_notifiers() into the section with IRQs disabled. In doing
> so, the commit somewhat inadvertently fixed the underlying issue that
> was papered over by commit 1650b4ebc99d ("KVM: Disable irq while
> unregistering user notifier"). I.e. in practice, the code and comment
> has been stale since commit a377ac1cd9d7b.
>
> Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
> [sean: rewrite changelog after rebasing, drop lockdep assert]
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Kai Huang <kai.huang@intel.com>

> ---
> arch/x86/kvm/x86.c | 14 +++-----------
> 1 file changed, 3 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index c927326344b1..719a5fa45eb1 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -602,18 +602,10 @@ static void kvm_on_user_return(struct user_return_notifier *urn)
> struct kvm_user_return_msrs *msrs
> = container_of(urn, struct kvm_user_return_msrs, urn);
> struct kvm_user_return_msr_values *values;
> - unsigned long flags;
>
> - /*
> - * Disabling irqs at this point since the following code could be
> - * interrupted and executed through kvm_arch_disable_virtualization_cpu()
> - */
> - local_irq_save(flags);
> - if (msrs->registered) {
> - msrs->registered = false;
> - user_return_notifier_unregister(urn);
> - }
> - local_irq_restore(flags);
> + msrs->registered = false;
> + user_return_notifier_unregister(urn);
> +
> for (slot = 0; slot < kvm_nr_uret_msrs; ++slot) {
> values = &msrs->values[slot];
> if (values->host != values->curr) {