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

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Message-ID: <20241210134827.GG2347147@nvidia.com>
Date: Tue, 10 Dec 2024 09:48:27 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Junxian Huang <huangjunxian6@...ilicon.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 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.

> 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