linux-kernel - Re: [PATCH 2/3] cxl/mbox: Add GET_POISON

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Date:   Fri, 17 Jun 2022 09:29:35 -0700
From:   Alison Schofield <alison.schofield@...el.com>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc:     "Williams, Dan J" <dan.j.williams@...el.com>,
        "Weiny, Ira" <ira.weiny@...el.com>,
        "Verma, Vishal L" <vishal.l.verma@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        "linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] cxl/mbox: Add GET_POISON_LIST mailbox command support

On Fri, Jun 17, 2022 at 07:05:08AM -0700, Jonathan Cameron wrote:
> On Tue, 14 Jun 2022 17:10:27 -0700
> alison.schofield@...el.com wrote:
> 
> > From: Alison Schofield <alison.schofield@...el.com>
> > 
> > CXL devices that support persistent memory maintain a list of locations
> > that are poisoned or result in poison if the addresses are accessed by
> > the host.
> > 
> > Per the spec (CXL 2.0 8.2.8.5.4.1), the device returns this Poison
> > list as a set of  Media Error Records that include the source of the
> > error, the starting device physical address and length. The length is
> > the number of adjacent DPAs in the record and is in units of 64 bytes.
> > 
> > Retrieve the list and log each Media Error Record as a trace event of
> > type cxl_poison_list.
> > 
> > Signed-off-by: Alison Schofield <alison.schofield@...el.com>
> 
> A few more things inline.
> 
> Otherwise, can confirm it works with some hack QEMU code.
> I'll tidy that up and post soon.
> 
> > +int cxl_mem_get_poison_list(struct device *dev)
> > +{
snip
> > +
> > +			trace_cxl_poison_list(dev, source, addr, len);
> 
> Need to mask off the lower 6 bits of addr as they contain the source
> + a few reserved bits.
> 
> I was confused how you were geting better than 64 byte precision in your
> example.
>
Ah...got it. Thanks!

> > +		}
> > +
> > +		/* Protect against an uncleared _FLAG_MORE */
> > +		nr_records = nr_records + le16_to_cpu(po->count);
> > +		if (nr_records >= cxlds->poison_max)
> > +			goto out;
> > +
> > +	} while (po->flags & CXL_POISON_FLAG_MORE);
> So.. A conundrum here.  What happens if:
> 
> 1. We get an error mid way through a set of multiple reads
>    (something intermittent - maybe a software issue)
> 2. We will drop out of here fine and report the error.
> 3. We run this function again.
> 
> It will (I think) currently pick up where we left off, but we have
> no way of knowing that as there isn't a 'total records' count or
> any other form of index in the output payload.

Yes. That is sad. I'm assume it's by design and CXL devices never
intended to keep any totals.

> 
> So, software solutions I think should work (though may warrant a note
> to be added to the spec).
> 
> 1. Read whole thing twice. First time is just to ensure we get
>    to the end and flush out any prior half done reads.
> 2. Issue a read for a different region (perhaps length 0) first
>    and assume everything starts from scratch when we go back to
>    this region.

Can you tell me more about 2 ?

Also, Since posting this I have added protection to this path to ensure
only one reader of the poison list for this device. Like this:

if (!completion_done(&cxlds->read_poison_complete);
              return -EBUSY;
wait_for_completion_interruptible(&cxlds->read_poison_complete);
	...GET ALL THE POISON...
complete(&cxlds->read_poison_complete);

And will add the error message on that unexpected _FLAG_MORE too.

Alison
> 
> Jonathan
> 



> > +
> > +out:
> > +	kvfree(po);
> > +	return rc;
> > +}
> > +EXPORT_SYMBOL_NS_GPL(cxl_mem_get_poison_list, CXL);
> > +
> >  struct cxl_dev_state *cxl_dev_state_create(struct device *dev)
> >  {
> >  	struct cxl_dev_state *cxlds;
>