linux-kernel - Re: [PATCH] KVM: x86: Check irqchip mode before create PIT

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Date: Wed, 24 Jan 2024 08:32:17 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Brilliant Hanabi <moehanabichan@...il.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Check irqchip mode before create PIT

On Thu, Jan 25, 2024, Brilliant Hanabi wrote:
> As the kvm api(https://docs.kernel.org/virt/kvm/api.html) reads,
> KVM_CREATE_PIT2 call is only valid after enabling in-kernel irqchip
> support via KVM_CREATE_IRQCHIP.
> 
> Without this check, I can create PIT first and enable irqchip-split
> then, which may cause the PIT invalid because of lacking of in-kernel
> PIC to inject the interrupt.

Does this cause actual problems beyond the PIT not working for the guest?  E.g.
does it put the host kernel at risk?  If the only problem is that the PIT doesn't
work as expected, I'm tempted to tweak the docs to say that KVM's PIT emulation
won't work without an in-kernel I/O APIC.  Rejecting the ioctl could theoertically
break misconfigured setups that happen to work, e.g. because the guest never uses
the PIT.

> Signed-off-by: Brilliant Hanabi <moehanabichan@...il.com>
> ---
>  arch/x86/kvm/x86.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 27e23714e960..3edc8478310f 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7016,6 +7016,8 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
>  		r = -EEXIST;
>  		if (kvm->arch.vpit)
>  			goto create_pit_unlock;
> +		if (!pic_in_kernel(kvm))
> +			goto create_pit_unlock;

-EEXIST is not an appropriate errno.

>  		r = -ENOMEM;
>  		kvm->arch.vpit = kvm_create_pit(kvm, u.pit_config.flags);
>  		if (kvm->arch.vpit)
> -- 
> 2.39.3
>