From: Auger Eric <email@example.com>
To: Marc Zyngier <firstname.lastname@example.org>,
Shaokun Zhang <email@example.com>
Alex Williamson <firstname.lastname@example.org>,
Cornelia Huck <email@example.com>,
Nianyao Tang <firstname.lastname@example.org>,
Bjorn Helgaas <email@example.com>
Subject: Re: Question on guest enable msi fail when using GICv4/4.1
Date: Sun, 9 May 2021 19:00:04 +0200 [thread overview]
Message-ID: <firstname.lastname@example.org> (raw)
On 5/7/21 1:02 PM, Marc Zyngier wrote:
> On Fri, 07 May 2021 10:58:23 +0100,
> Shaokun Zhang <email@example.com> wrote:
>> Hi Marc,
>> Thanks for your quick reply.
>> On 2021/5/7 17:03, Marc Zyngier wrote:
>>> On Fri, 07 May 2021 06:57:04 +0100,
>>> Shaokun Zhang <firstname.lastname@example.org> wrote:
>>>> [This letter comes from Nianyao Tang]
>>>> Using GICv4/4.1 and msi capability, guest vf driver requires 3
>>>> vectors and enable msi, will lead to guest stuck.
>>> Stuck how?
>> Guest serial does not response anymore and guest network shutdown.
>>>> Qemu gets number of interrupts from Multiple Message Capable field
>>>> set by guest. This field is aligned to a power of 2(if a function
>>>> requires 3 vectors, it initializes it to 2).
>>> So I guess this is a MultiMSI device with 4 vectors, right?
>> Yes, it can support maximum of 32 msi interrupts, and vf driver only use 3 msi.
>>>> However, guest driver just sends 3 mapi-cmd to vits and 3 ite
>>>> entries is recorded in host. Vfio initializes msi interrupts using
>>>> the number of interrupts 4 provide by qemu. When it comes to the
>>>> 4th msi without ite in vits, in irq_bypass_register_producer,
>>>> producer and consumer will __connect fail, due to find_ite fail, and
>>>> do not resume guest.
>>> Let me rephrase this to check that I understand it:
>>> - The device has 4 vectors
>>> - The guest only create mappings for 3 of them
>>> - VFIO calls kvm_vgic_v4_set_forwarding() for each vector
>>> - KVM doesn't have a mapping for the 4th vector and returns an error
>>> - VFIO disable this 4th vector
>>> Is that correct? If yes, I don't understand why that impacts the guest
>>> at all. From what I can see, vfio_msi_set_vector_signal() just prints
>>> a message on the console and carries on.
>> function calls:
>> --> vfio_msi_set_vector_signal
>> --> irq_bypass_register_producer
>> in __connect, add_producer finally calls kvm_vgic_v4_set_forwarding
>> and fails to get the 4th mapping. When add_producer fail, it does
>> not call cons->start, calls kvm_arch_irq_bypass_start and then
> [+Eric, who wrote the irq_bypass infrastructure.]
> Ah, so the guest is actually paused, not in a livelock situation
> (which is how I interpreted "stuck").
> I think we should handle this case gracefully, as there should be no
> expectation that the guest will be using this interrupt. Given that
> VFIO seems to be pretty unfazed when a producer fails, I'm temped to
> do the same thing and restart the guest.
> Also, __disconnect doesn't care about errors, so why should __connect
> have this odd behaviour?
_disconnect() does not care as we should always succeed tearing off
things. del_* ops are void functions. On the opposite we can fail
setting up the bypass.
a979a6aa009f ("irqbypass: do not start cons/prod when failed connect")
needs to be reverted.
I agree the kerneldoc comments in linux/irqbypass.h may be improved to
better explain the role of stop/start cbs and warn about their potential
wrt the case above, "in __connect, add_producer finally calls
kvm_vgic_v4_set_forwarding and fails to get the 4th mapping", shouldn't
we succeed in that case?
> Can you please try this? It is completely untested (and I think the
> del_consumer call is odd, which is why I've also dropped it).
> Eric, what do you think?
> diff --git a/virt/lib/irqbypass.c b/virt/lib/irqbypass.c
> index c9bb3957f58a..7e1865e15668 100644
> --- a/virt/lib/irqbypass.c
> +++ b/virt/lib/irqbypass.c
> @@ -40,21 +40,14 @@ static int __connect(struct irq_bypass_producer *prod,
> if (prod->add_consumer)
> ret = prod->add_consumer(prod, cons);
> - if (ret)
> - goto err_add_consumer;
> - ret = cons->add_producer(cons, prod);
> - if (ret)
> - goto err_add_producer;
> + if (!ret)
> + ret = cons->add_producer(cons, prod);
> if (cons->start)
> if (prod->start)
> - if (prod->del_consumer)
> - prod->del_consumer(prod, cons);
> return ret;
next prev parent reply other threads:[~2021-05-09 17:00 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-07 5:57 Question on guest enable msi fail when using GICv4/4.1 Shaokun Zhang
2021-05-07 9:03 ` Marc Zyngier
2021-05-07 9:58 ` Shaokun Zhang
2021-05-07 11:02 ` Marc Zyngier
[not found] ` <email@example.com>
2021-05-08 1:51 ` Jason Wang
2021-05-08 6:56 ` Zhu, Lingshan
2021-05-08 9:15 ` Marc Zyngier
2021-05-09 17:00 ` Auger Eric [this message]
2021-05-10 7:49 ` Marc Zyngier
2021-05-10 8:29 ` Auger Eric
2021-05-10 9:59 ` Marc Zyngier
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