linux-kernel - Re: [PATCH V13 8/9] cxl/port: Retry reading CDAT on failure

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Date:   Thu, 14 Jul 2022 13:13:41 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        "Alison Schofield" <alison.schofield@...el.com>,
        Lukas Wunner <lukas@...ner.de>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        <linux-kernel@...r.kernel.org>, <linux-cxl@...r.kernel.org>,
        <linux-pci@...r.kernel.org>
Subject: Re: [PATCH V13 8/9] cxl/port: Retry reading CDAT on failure

On Thu, Jul 14, 2022 at 01:05:47PM -0700, Ira wrote:
> On Thu, Jul 14, 2022 at 09:27:04AM -0700, Dan Williams wrote:
> > ira.weiny@ wrote:
> > > From: Ira Weiny <ira.weiny@...el.com>
> > > 
> > > The CDAT read may fail for a number of reasons but mainly it is possible
> > > to get different parts of a valid state.  The checksum in the CDAT table
> > > protects against this.
> > 
> > I don't know what "different parts of a valid state" means.
> 
> This text is stale but given what I know about how other entities may be
> issuing queries without the kernel knowledge I'm not 100% sure that the data
> read back will always be valid.
> 
> Regardless, this has already caught a bug in QEMU.
> 
> So I'm inclined to leave this check in because the checksum is there and should
> can be validated if only to detect broken hardware.
> 
> I can update the commit message to clarify this.

Oh wait I thought this was the 'is valid' patch.

I can remove the retries if that was all you were concerned about.

Ira

> 
> Ira
> 
> >
> > The CDAT
> > should not be changing as it is being read unless someone is issuing a
> > set-partition while the DOE operation is happening. Rather than
> > arbitrary retries, block out set-partition while CDAT is being read.
> > 
> > You can use {set,clear}_exclusive_cxl_commands() to temporarily lock out
> > set-partition while the CDAT read is happening.
> > 
> > ...and since this series is only for enabling