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From: Yi Wang <up2wing@gmail.com>
Date: Wed, 17 Jan 2024 08:46:49 +0800
Message-ID: <CAN35MuTOWYvboZtk_dQXEQ_+vDEO+ao9pzxLSkJj3x8RboAsSw@mail.gmail.com>
Subject: Re: [PATCH] KVM: irqchip: synchronize srcu only if needed
To: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Sean Christopherson <seanjc@google.com>, pbonzini@redhat.com, tglx@linutronix.de, 
	mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, 
	hpa@zytor.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, 
	wanpengli@tencent.com, Yi Wang <foxywang@tencent.com>, 
	Oliver Upton <oliver.upton@linux.dev>, Marc Zyngier <maz@kernel.org>, 
	Anup Patel <anup@brainfault.org>, Atish Patra <atishp@atishpatra.org>, 
	Janosch Frank <frankja@linux.ibm.com>, Claudio Imbrenda <imbrenda@linux.ibm.com>
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On Wed, Jan 17, 2024 at 12:50=E2=80=AFAM Christian Borntraeger
<borntraeger@linux.ibm.com> wrote:
>
>
>
> Am 15.01.24 um 17:01 schrieb Yi Wang:
> > Many thanks for your such kind and detailed reply, Sean!
> >

...

> >>
> >> So instead of special casing x86, what if we instead have KVM setup an=
 empty
> >> IRQ routing table during kvm_create_vm(), and then avoid this mess ent=
irely?
> >> That way x86 and s390 no longer need to set empty/dummy routing when c=
reating
> >> an IRQCHIP, and the worst case scenario of userspace misusing an ioctl=
() is no
> >> longer a NULL pointer deref.
>
> Sounds like a good idea. This should also speedup guest creation on s390 =
since
> it would avoid one syncronize_irq.
> >
> > To setup an empty IRQ routing table during kvm_create_vm() sounds a goo=
d idea,
> > at this time vCPU have not been created and kvm->lock is held so skippi=
ng
> > synchronization is safe here.
> >
> > However, there is one drawback, if vmm wants to emulate irqchip
> > itself, e.g. qemu
> > with command line '-machine kernel-irqchip=3Doff' may not need irqchip
> > in kernel. How
> > do we handle this issue?
>
> I would be fine with wasted memory. The only question is does it have a f=
unctional
> impact or can we simply ignore the dummy routing.
>

Thanks for your reply, I will update the patch.


--=20
---
Best wishes
Yi Wang