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

[Ira Weiny <ira.weiny@intel.com>]

On Thu, Jul 14, 2022 at 09:27:04AM -0700, Dan Williams wrote:
> ira.weiny@ wrote:
> > From: Ira Weiny <ira.weiny@intel.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