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

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Message-ID: <3107893f25d6c4cb32b8be9a78c26c721cc6019f.camel@intel.com>
Date: Tue, 4 Nov 2025 10:34:23 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "pbonzini@...hat.com" <pbonzini@...hat.com>, "kas@...nel.org"
	<kas@...nel.org>, "seanjc@...gle.com" <seanjc@...gle.com>
CC: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, "x86@...nel.org"
	<x86@...nel.org>, "hou, wenlong" <houwenlong.hwl@...group.com>, "Li, Xiaoyao"
	<xiaoyao.li@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-coco@...ts.linux.dev"
	<linux-coco@...ts.linux.dev>
Subject: Re: [PATCH v5 4/4] KVM: x86: Don't disable IRQs when unregistering
 user-return notifier

On Thu, 2025-10-30 at 12:15 -0700, Sean Christopherson wrote:
> From: Hou Wenlong <houwenlong.hwl@...group.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@...group.com>
> [sean: rewrite changelog after rebasing, drop lockdep assert]
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Kai Huang <kai.huang@...el.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) {