Re: [PATCH] KVM: irqchip: synchronize srcu only if needed

[Sean Christopherson <seanjc@google.com>]

+other KVM maintainers

On Fri, Jan 12, 2024, Yi Wang wrote:
> From: Yi Wang <foxywang@tencent.com>
> 
> We found that it may cost more than 20 milliseconds very accidentally
> to enable cap of KVM_CAP_SPLIT_IRQCHIP on a host which has many vms
> already.
> 
> The reason is that when vmm(qemu/CloudHypervisor) invokes
> KVM_CAP_SPLIT_IRQCHIP kvm will call synchronize_srcu_expedited() and
> might_sleep and kworker of srcu may cost some delay during this period.

might_sleep() yielding is not justification for changing KVM.  That's more or
less saying "my task got preempted and took longer to run".  Well, yeah.

> Since this happens during creating vm, it's no need to synchronize srcu
> now 'cause everything is not ready(vcpu/irqfd) and none uses irq_srcu now.
> 
> Signed-off-by: Yi Wang <foxywang@tencent.com>
> ---
>  arch/x86/kvm/irq_comm.c  | 2 +-
>  include/linux/kvm_host.h | 2 ++
>  virt/kvm/irqchip.c       | 3 ++-
>  3 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/irq_comm.c b/arch/x86/kvm/irq_comm.c
> index 16d076a1b91a..37c92b7486c7 100644
> --- a/arch/x86/kvm/irq_comm.c
> +++ b/arch/x86/kvm/irq_comm.c
> @@ -394,7 +394,7 @@ static const struct kvm_irq_routing_entry empty_routing[] = {};
>  
>  int kvm_setup_empty_irq_routing(struct kvm *kvm)
>  {
> -	return kvm_set_irq_routing(kvm, empty_routing, 0, 0);
> +	return kvm_set_irq_routing(kvm, empty_routing, 0, NONEED_SYNC_SRCU);
>  }
>  
>  void kvm_arch_post_irq_routing_update(struct kvm *kvm)
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 4944136efaa2..a46370cca355 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1995,6 +1995,8 @@ static inline int mmu_invalidate_retry_hva(struct kvm *kvm,
>  
>  #define KVM_MAX_IRQ_ROUTES 4096 /* might need extension/rework in the future */
>  
> +#define NONEED_SYNC_SRCU	(1U << 0)
> +
>  bool kvm_arch_can_set_irq_routing(struct kvm *kvm);
>  int kvm_set_irq_routing(struct kvm *kvm,
>  			const struct kvm_irq_routing_entry *entries,
> diff --git a/virt/kvm/irqchip.c b/virt/kvm/irqchip.c
> index 1e567d1f6d3d..cea5c43c1a49 100644
> --- a/virt/kvm/irqchip.c
> +++ b/virt/kvm/irqchip.c
> @@ -224,7 +224,8 @@ int kvm_set_irq_routing(struct kvm *kvm,
>  
>  	kvm_arch_post_irq_routing_update(kvm);
>  
> -	synchronize_srcu_expedited(&kvm->irq_srcu);
> +	if (!(flags & NONEED_SYNC_SRCU))
> +		synchronize_srcu_expedited(&kvm->irq_srcu);

I'm not a fan of x86 passing in a magic flag.  It's not immediately clear why
skipping synchronization is safe.  Piecing things together, _on x86_, I believe
the answer is that vCPU can't have yet been created, kvm->lock is held, _and_
kvm_arch_irqfd_allowed() will subtly reject attempts to assign irqfds if the local
APIC isn't in-kernel.

But AFAICT, s390's implementation of KVM_CREATE_IRQCHIP, which sets up identical
dummy/empty routing, doesn't provide the same guarantees.

	case KVM_CREATE_IRQCHIP: {
		struct kvm_irq_routing_entry routing;

		r = -EINVAL;
		if (kvm->arch.use_irqchip) {
			/* Set up dummy routing. */
			memset(&routing, 0, sizeof(routing));
			r = kvm_set_irq_routing(kvm, &routing, 0, 0);
		}
		break;
	}

It's entirely possible that someday, kvm_setup_empty_irq_routing() is moved to
common KVM and used for s390, at which point we have a mess on our hands because
it's not at all obvious whether or not it's safe for s390 to also skip
synchronization.

Rather than hack in a workaround for x86, I would rather we try and clean up this
mess.

Except for kvm_irq_map_gsi(), it looks like all flows assume irq_routing is
non-NULL.  But I'm not remotely confident that that holds true on all architectures,
e.g. the only reason kvm_irq_map_gsi() checks for a NULL irq_routing is because
syzkaller generated a splat (commit c622a3c21ede ("KVM: irqfd: fix NULL pointer
dereference in kvm_irq_map_gsi")).

And on x86, I'm pretty sure as of commit 654f1f13ea56 ("kvm: Check irqchip mode
before assign irqfd"), which added kvm_arch_irqfd_allowed(), it's impossible for
kvm_irq_map_gsi() to encounter a NULL irq_routing _on x86_.

But I strongly suspect other architectures can reach kvm_irq_map_gsi() with a
NULL irq_routing, e.g. RISC-V dynamically configures its interrupt controller,
yet doesn't implement kvm_arch_intc_initialized().

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 entirely?
That way x86 and s390 no longer need to set empty/dummy routing when creating
an IRQCHIP, and the worst case scenario of userspace misusing an ioctl() is no
longer a NULL pointer deref.