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Date: Wed, 24 Jan 2024 09:43:58 -0800
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Subject: Re: Re: [PATCH] KVM: x86: Check irqchip mode before create PIT
From: Sean Christopherson <seanjc@google.com>
To: moehanabi <moehanabichan@gmail.com>
Cc: bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, 
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org, mingo@redhat.com, 
	pbonzini@redhat.com, tglx@linutronix.de, x86@kernel.org
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On Thu, Jan 25, 2024, moehanabi wrote:
> > 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.
> 
> I don't think it will put the host kernel at risk. But that's exactly what
> kvmtool does: it creates in-kernel PIT first and set KVM_CREATE_IRQCHIP then.

Right.  My concern, which could be unfounded paranoia, is that rejecting an ioctl()
that used to succeed could break existing setups.  E.g. if a userspace VMM creates
a PIT and checks the ioctl() result, but its guest(s) never actually use the PIT
and so don't care that the PIT is busted.

> I found this problem because I was working on implementing a userspace PIC
> and PIT in kvmtool. As I planned, I'm going to commit a related patch to 
> kvmtool if this patch will be applied.
> 
> > > Signed-off-by: Brilliant Hanabi <moehanabichan@gmail.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.
> 
> Which errno do you think is better?

Maybe ENOENT?