linux-kernel - Re: [PATCH 00/15] cxl: Add support for Restricted CXL hosts (RCD mode)

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Message-ID: <17F02AD3-D68A-4E6A-A549-A47611B335E7@fb.com>
Date:   Thu, 8 Sep 2022 18:52:46 +0000
From:   "Jonathan Zhang (Infra)" <jonzhang@...com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     Robert Richter <rrichter@....com>,
        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-cxl@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>
Subject: Re: [PATCH 00/15] cxl: Add support for Restricted CXL hosts (RCD
 mode)



> On Sep 7, 2022, at 10:43 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> 
> Apologies for the delay in getting to this I had hoped to be able to
> finish up some other DAX work to focus on this, but time is getting
> short so I will need to do both in parallel.
> 
> Robert Richter wrote:
>> In Restricted CXL Device (RCD) mode (formerly referred to as CXL 1.1)
>> the PCIe enumeration hierarchy is different from CXL VH Enumeration
>> (formerly referred to as 2.0, for both modes see CXL spec 3.0: 9.11
>> and 9.12, [1]). This series adds support for RCD mode. It implements
>> the detection of Restricted CXL Hosts (RCHs) and its corresponding
>> Restricted CXL Devices (RCDs). It does the necessary enumeration of
>> ports and connects the endpoints. With all the plumbing an RCH/RCD
>> pair is registered at the Linux CXL bus and becomes visible in sysfs
>> in the same way as CXL VH hosts and devices do already. RCDs are
>> brought up as CXL endpoints and bound to subsequent drivers such as
>> cxl_mem.
>> 
>> For CXL VH the host driver (cxl_acpi) starts host bridge discovery
>> once the ACPI0017 CXL root device is detected and then searches for
>> ACPI0016 host bridges to enable CXL. In RCD mode an ACPI0017 device
>> might not necessarily exist 
> 
> That's a broken BIOS as far as I can see. No ACPI0017 == no OS CXL
> services and the CXL aspects of the device need to be 100% managed by
> the BIOS. You can still run the cxl_pci driver in that case for mailbox
> operation, but error handling must be firmware-first without ACPI0017.
Firmware-first or OS-first applies to CXL protocol error handling. For CXL 
memory error handling, the device generates a DRAM error record, the OS
parses such record and act accordingly. According to CXL spec (section
8.2.9.2.1.2 DRAM Event Record), DPA but not HPA is in such record. The OS
needs to translate such DPA into HPA to act on. I am taking this as an example
to show that OS CXL services is needed.
Instead of using ACPI0016 to tell whether the system is under RCH mode,
I suppose one way is to check “CXL version” field of CHBS structure in CEDT?

> 
>> PCIe host bridge PNP0A08 ID, there aren't any CXL port or switches in
>> the PCIe hierarchy visible. As such the RCD mode enumeration and host
>> discovery is very different from CXL VH. See patch #5 for
>> implementation details.
>> 
>> This implementation expects the host's downstream and upstream port
>> RCRBs base address being reported by firmware using the optional CEDT
>> CHBS entry of the host bridge (see CXL spec 3.0, 9.17.1.2).
>> 
>> RCD mode does not support hot-plug, so host discovery is at boot time
>> only.
>> 
>> Patches #1 to #4 are prerequisites of the series with fixes needed and
>> a rework of debug messages for port enumeration. Those are general
>> patches and could be applied earlier and independently from the rest
>> assuming there are no objections with them. Patches #5 to #15 contain
>> the actual implementation of RCD mode support.
>> 
>> [1] https://www.computeexpresslink.org/spec-landing
>> 
>> Robert Richter (15):
>>  cxl/core: Remove duplicate declaration of devm_cxl_iomap_block()
>>  cxl/core: Check physical address before mapping it in
>>    devm_cxl_iomap_block()
>>  cxl: Unify debug messages when calling devm_cxl_add_port()
>>  cxl: Unify debug messages when calling devm_cxl_add_dport()
>>  cxl/acpi: Add probe function to detect restricted CXL hosts in RCD
>>    mode
>>  PCI/ACPI: Link host bridge to its ACPI fw node
>>  cxl/acpi: Check RCH's PCIe Host Bridge ACPI ID
>>  cxl/acpi: Check RCH's CXL DVSEC capabilities
>>  cxl/acpi: Determine PCI host bridge's ACPI UID
>>  cxl/acpi: Extract the RCH's RCRB base address from CEDT
>>  cxl/acpi: Extract the host's component register base address from RCRB
>>  cxl/acpi: Skip devm_cxl_port_enumerate_dports() when in RCD mode
>>  cxl/acpi: Rework devm_cxl_enumerate_ports() to support RCD mode
>>  cxl/acpi: Enumerate ports in RCD mode to enable RCHs and RCDs
>>  cxl/acpi: Specify module load order dependency for the cxl_acpi module
>> 
>> drivers/acpi/pci_root.c      |   1 +
>> drivers/cxl/acpi.c           | 311 ++++++++++++++++++++++++++++++++++-
>> drivers/cxl/core/pci.c       |  22 ++-
>> drivers/cxl/core/port.c      | 103 ++++++++----
>> drivers/cxl/core/regs.c      |   3 +
>> drivers/cxl/cxl.h            |   2 -
>> drivers/cxl/mem.c            |   1 +
>> tools/testing/cxl/test/cxl.c |   8 +-
>> 8 files changed, 400 insertions(+), 51 deletions(-)
>> 
>> -- 
>> 2.30.2
>> 
> 
> 
>