From: Jason Wang <firstname.lastname@example.org>
To: Marc Zyngier <email@example.com>,
Zhu Lingshan <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>,
Eric Auger <firstname.lastname@example.org>,
"Michael S. Tsirkin" <email@example.com>
Subject: Re: Question on guest enable msi fail when using GICv4/4.1
Date: Sat, 8 May 2021 09:51:39 +0800 [thread overview]
Message-ID: <firstname.lastname@example.org> (raw)
在 2021/5/8 上午1:36, Marc Zyngier 写道:
> On Fri, 07 May 2021 12:02:57 +0100,
> Marc Zyngier <email@example.com> wrote:
>> On Fri, 07 May 2021 10:58:23 +0100,
>> Shaokun Zhang <firstname.lastname@example.org> 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 <email@example.com> 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?
>> 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?
> Adding Zhu, Jason, MST to the party. It all seems to be caused by this
> commit a979a6aa009f3c99689432e0cdb5402a4463fb88
> Author: Zhu Lingshan <firstname.lastname@example.org>
> Date: Fri Jul 31 14:55:33 2020 +0800
> irqbypass: do not start cons/prod when failed connect
> If failed to connect, there is no need to start consumer nor
> Signed-off-by: Zhu Lingshan <email@example.com>
> Suggested-by: Jason Wang <firstname.lastname@example.org>
> Link: https://email@example.com
> Signed-off-by: Michael S. Tsirkin <firstname.lastname@example.org>
> Zhu, I'd really like to understand why you think it is OK not to
> restart consumer and producers when a connection has failed to be
> established between the two?
My bad, I didn't check ARM code but it's not easy to infer that the
cons->start/stop is not a per consumer specific operation but a global
one like VM halting/resuming.
> In the case of KVM/arm64, this results in the guest being forever
> suspended and never resumed. That's obviously not an acceptable
> regression, as there is a number of benign reasons for a connect to
Let's revert this commit.
next prev parent reply other threads:[~2021-05-08 1:52 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 [this message]
2021-05-08 6:56 ` Zhu, Lingshan
2021-05-08 9:15 ` Marc Zyngier
2021-05-09 17:00 ` Auger Eric
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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