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

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Message-ID: <YtB3GySfcm5CfG01@iweiny-desk3>
Date:   Thu, 14 Jul 2022 13:05:47 -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 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.

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