linux-kernel - Re: [PATCH v3 2/9] cxl/acpi: Extract component registers of restricted hosts from RCRB

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Date:   Fri, 18 Nov 2022 09:12:21 +0100
From:   Robert Richter <rrichter@....com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        <linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        "Jonathan Cameron" <Jonathan.Cameron@...wei.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Dave Jiang <dave.jiang@...el.com>,
        Terry Bowman <terry.bowman@....com>
Subject: Re: [PATCH v3 2/9] cxl/acpi: Extract component registers of
 restricted hosts from RCRB

On 17.11.22 11:23:16, Dan Williams wrote:
> Robert Richter wrote:
> > On 17.11.22 09:20:55, Dan Williams wrote:
> > > Robert Richter wrote:
> > > > On 15.11.22 09:54:16, Dan Williams wrote:
> > > > > Robert Richter wrote:
> > > > > > On 14.11.22 13:30:01, Dan Williams wrote:

> > > > > Oh, sorry, yes, my mistake. However, there is not much value in mapping
> > > > > less than 4K since all ioremap requests are rounded up to PAGE_SIZE.
> > > > > Since an RCRB is only 4K per port lets just map the whole thing.
> > > > 
> > > > I was going to keep the ranges small to avoid conflicts with other
> > > > requests for the same page (though request_mem_region() was missing
> > > > yet).
> > > 
> > > What else will be conflicting the RCRB? Linux has never accessed an RCRB
> > > in the past as far as I can see. If there is a conflict then we may need
> > > to move this mapping to the PCI core so that it is managed like other
> > > mmconf space.
> > 
> > The capabilities (PCIe and DVSEC) could be used by various subsystems
> > and parts of the driver. I am thinking of the various RAS caps (UP,
> > DP, CXL, AER variants) that are accessed from different parts of the
> > driver. Of curse, access could be delegated but else there is the
> > option to directly map and access that parts. In the component reg
> > block we already see issues with that broad mappings.
> 
> Sure, but lets cross that bridge when we get to that point. Something is
> broken if these competing usages can not at least have their own page
> mapping since that limits being able to hand out control across security
> boundaries (like VMs or userspace). Any ioremap less than PAGE_SIZE is
> somewhat suspect.
> 
> The cxl_port driver so far seems to be sufficient for owning the entire
> component register space.

Ok, I can change that.

Thanks,

-Robert