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

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

[Yi Wang]

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.
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);
 
 	new = old;
 	r = 0;
-- 
2.39.3

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

[Sean Christopherson]

+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.

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

[Yi Wang]

Many thanks for your such kind and detailed reply, Sean!

On Sat, Jan 13, 2024 at 12:28 AM Sean Christopherson <seanjc@google.com> wrote:
>
> +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.

Agree. But I suppose it may be one of the reasons that makes  time of
KVM_CAP_SPLIT_IRQCHIP delayed, of course, the kworker has the biggest
suspicion :)

>
> > 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.

....

> 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.

To setup an empty IRQ routing table during kvm_create_vm() sounds a good idea,
at this time vCPU have not been created and kvm->lock is held so skipping
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=off' may not need irqchip
in kernel. How
do we handle this issue?


---
Best wishes
Yi Wang

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

[Christian Borntraeger]

Am 15.01.24 um 17:01 schrieb Yi Wang:
> Many thanks for your such kind and detailed reply, Sean!
> 
> On Sat, Jan 13, 2024 at 12:28 AM Sean Christopherson <seanjc@google.com> wrote:
>>
>> +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.
> 
> Agree. But I suppose it may be one of the reasons that makes  time of
> KVM_CAP_SPLIT_IRQCHIP delayed, of course, the kworker has the biggest
> suspicion :)
> 
>>
>>> 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.
> 
> ....
> 
>> 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.

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 good idea,
> at this time vCPU have not been created and kvm->lock is held so skipping
> 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=off' 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 functional
impact or can we simply ignore the dummy routing.

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

[Sean Christopherson]

On Tue, Jan 16, 2024, Christian Borntraeger wrote:
> 
> 
> Am 15.01.24 um 17:01 schrieb Yi Wang:
> > Many thanks for your such kind and detailed reply, Sean!
> > 
> > On Sat, Jan 13, 2024 at 12:28 AM Sean Christopherson <seanjc@google.com> wrote:
> > > 
> > > +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.
> > 
> > Agree. But I suppose it may be one of the reasons that makes  time of
> > KVM_CAP_SPLIT_IRQCHIP delayed, of course, the kworker has the biggest
> > suspicion :)
> > 
> > > 
> > > > 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.
> > 
> > ....
> > 
> > > 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.
> 
> 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 good idea,
> > at this time vCPU have not been created and kvm->lock is held so skipping
> > 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=off' may not need
> > irqchip in kernel. How do we handle this issue?
> 
> I would be fine with wasted memory.

+1.  If we really, really want to avoid the negligible memory overhead, we could
pre-configure a static global table and directly use that as the dummy table (and
exempt it from being freed by free_irq_routing_table()).

> The only question is does it have a functional impact or can we simply ignore
> the dummy routing.

Given the lack of sanity checks on kvm->irq_routing, I'm pretty sure the only way
for there to be functional impact is if there's a latent NULL pointer deref hiding
somewhere.

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

[Yi Wang]

On Wed, Jan 17, 2024 at 12:58 AM Sean Christopherson <seanjc@google.com> wrote:
>
> On Tue, Jan 16, 2024, Christian Borntraeger wrote:
> >
> >

....

> >
> > I would be fine with wasted memory.
>
> +1.  If we really, really want to avoid the negligible memory overhead, we could
> pre-configure a static global table and directly use that as the dummy table (and
> exempt it from being freed by free_irq_routing_table()).

Thanks for the suggestion! Well, in my opinion it may be better to fix
the current issue
and I'm glad to send another patch to optimize this.

> > The only question is does it have a functional impact or can we simply ignore
> > the dummy routing.
>
> Given the lack of sanity checks on kvm->irq_routing, I'm pretty sure the only way
> for there to be functional impact is if there's a latent NULL pointer deref hiding
> somewhere.


-- 
---
Best wishes
Yi Wang

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

[Yi Wang]

On Wed, Jan 17, 2024 at 12:50 AM 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 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.
>
> 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 good idea,
> > at this time vCPU have not been created and kvm->lock is held so skipping
> > 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=off' 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 functional
> impact or can we simply ignore the dummy routing.
>

Thanks for your reply, I will update the patch.


-- 
---
Best wishes
Yi Wang