linux-kernel - Re: [PATCH for-next] RDMA/hns: Support mmapping reset state to userspace

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Message-ID: <92d90a3b-40e5-b47a-e570-ca63e4e58126@hisilicon.com>
Date: Fri, 13 Dec 2024 17:37:58 +0800
From: Junxian Huang <huangjunxian6@...ilicon.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: <leon@...nel.org>, <linux-rdma@...r.kernel.org>, <linuxarm@...wei.com>,
	<linux-kernel@...r.kernel.org>, <tangchengchang@...wei.com>
Subject: Re: [PATCH for-next] RDMA/hns: Support mmapping reset state to
 userspace



On 2024/12/10 21:48, Jason Gunthorpe wrote:
> On Tue, Dec 10, 2024 at 02:24:16PM +0800, Junxian Huang wrote:
>>
>>
>> On 2024/12/10 3:01, Jason Gunthorpe wrote:
>>> On Mon, Oct 14, 2024 at 09:07:31PM +0800, Junxian Huang wrote:
>>>> From: Chengchang Tang <tangchengchang@...wei.com>
>>>>
>>>> Mmap reset state to notify userspace about HW reset. The mmaped flag
>>>> hw_ready will be initiated to a non-zero value. When HW is reset,
>>>> the mmap page will be zapped and userspace will get a zero value of
>>>> hw_ready.
>>>
>>> This needs alot more explanation about *why* does userspace need this
>>> information and why is hns unique here.
>>>
>>
>> Our HW cannot flush WQEs by itself unless the driver posts a modify-qp-to-err
>> mailbox. But when the HW is reset, it'll stop handling mailbox too, so the HW
>> becomes unable to produce any more CQEs for the existing WQEs. This will break
>> some users' expectation that they should be able to poll CQEs as many as the
>> number of the posted WQEs in any cases.
> 
> But your reset flow partially disassociates the device, when the
> userspace goes back to sleep, or rearms the CQ, it should get a hard
> fail and do a full cleanup without relying on flushing.
> 

Not sure if I got your point, when you said "the userspace goes back to sleep",
did you mean the ibv_get_async_event() api? Are you suggesting that userspace
should call ibv_get_async_event() to monitor async events, and when it gets a
fatal event, it should stop polling CQs and clean up everything instead of
still waiting for the remaining CQEs?

Thanks,
Junxian

>> We try to notify the reset state to userspace so that we can generate software
>> WCs for the existing WQEs in userspace instead of HW in reset state, which is
>> what this rdma-core PR does:
> 
> That doesn't sound right at all. Device disassociation is a hard fail,
> we don't try to elegantly do things like generate completions. The
> device is dead, the queues are gone.
> 
> Jason
>