linux-kernel - Re: [PATCH 03/14] cxl/mem: Find device capabilities

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Date:   Mon, 1 Feb 2021 15:17:18 -0800
From:   Ben Widawsky <ben.widawsky@...el.com>
To:     David Rientjes <rientjes@...gle.com>
Cc:     linux-cxl@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-pci@...r.kernel.org, Bjorn Helgaas <helgaas@...nel.org>,
        Chris Browy <cbrowy@...ry-design.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Jon Masters <jcm@...masters.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Vishal Verma <vishal.l.verma@...el.com>,
        daniel.lll@...baba-inc.com,
        "John Groves (jgroves)" <jgroves@...ron.com>,
        "Kelley, Sean V" <sean.v.kelley@...el.com>
Subject: Re: [PATCH 03/14] cxl/mem: Find device capabilities

On 21-02-01 15:09:45, David Rientjes wrote:
> On Mon, 1 Feb 2021, Ben Widawsky wrote:
> 
> > > I think that's what 8.2.8.4.3 says, no?  And then 8.2.8.4.5 says you 
> > > can use up to Payload Size.  That's why my recommendation was to enforce 
> > > this in cxl_mem_setup_mailbox() up front.
> > 
> > Yeah. I asked our spec people to update 8.2.8.4.5 to make it clearer. I'd argue
> > the intent is how you describe it, but the implementation isn't.
> > 
> > My argument was silly anyway because if you specify greater than 1M as your
> > payload, you will get EINVAL at the ioctl.
> > 
> > The value of how it works today is the driver will at least bind and allow you
> > to interact with it.
> > 
> > How strongly do you feel about this?
> > 
> 
> I haven't seen the update to 8.2.8.4.5 to know yet :)
> 
> You make a good point of at least being able to interact with the driver.  
> I think you could argue that if the driver binds, then the payload size is 
> accepted, in which case it would be strange to get an EINVAL when using 
> the ioctl with anything >1MB.
> 
> Concern was that if we mask off the reserved bits from the command 
> register that we could be masking part of the payload size that is being 
> passed if the accepted max is >1MB.  Idea was to avoid any possibility of 
> this inconsistency.  If this is being checked for ioctl, seems like it's 
> checking reserved bits.
> 
> But maybe I should just wait for the spec update.

Well, I wouldn't hold your breath (it would be an errata in this case anyway).
My preference would be to just allow allow mailbox payload size to be 2^31 and
not deal with this.

My question was how strongly do you feel it's an error that should prevent
binding.