From: "Zhu, Lingshan" <firstname.lastname@example.org>
To: Jason Wang <email@example.com>, 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>,
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 14:56:58 +0800 [thread overview]
Message-ID: <firstname.lastname@example.org> (raw)
On 5/8/2021 9:51 AM, Jason Wang wrote:
> 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
>>>>> 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>
>> Signed-off-by: Michael S. Tsirkin <email@example.com>
>> 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.
I will send out a patch to revert this commit as Jason suggested.
>> 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 6:57 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] ` <firstname.lastname@example.org>
2021-05-08 1:51 ` Jason Wang
2021-05-08 6:56 ` Zhu, Lingshan [this message]
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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