linux-kernel - [PATCH v2 8/9] hwmon: (dell-smm) Document the WMI SMM interface

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20231104155559.11842-9-W_Armin@gmx.de>
Date:   Sat,  4 Nov 2023 16:55:58 +0100
From:   Armin Wolf <W_Armin@....de>
To:     pali@...nel.org
Cc:     hdegoede@...hat.com, markgross@...nel.org,
        ilpo.jarvinen@...ux.intel.com, jdelvare@...e.com,
        linux@...ck-us.net, platform-driver-x86@...r.kernel.org,
        linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v2 8/9] hwmon: (dell-smm) Document the WMI SMM interface

Document the WMI SMM interface so that future developers
can better understand how it works.

Signed-off-by: Armin Wolf <W_Armin@....de>
---
 Documentation/hwmon/dell-smm-hwmon.rst | 38 ++++++++++++++++++++++++--
 1 file changed, 35 insertions(+), 3 deletions(-)

diff --git a/Documentation/hwmon/dell-smm-hwmon.rst b/Documentation/hwmon/dell-smm-hwmon.rst
index d8f1d6859b96..977263cb57a8 100644
--- a/Documentation/hwmon/dell-smm-hwmon.rst
+++ b/Documentation/hwmon/dell-smm-hwmon.rst
@@ -186,8 +186,7 @@ SMM Interface
 The driver uses the SMM interface to send commands to the system BIOS.
 This interface is normally used by Dell's 32-bit diagnostic program or
 on newer notebook models by the buildin BIOS diagnostics.
-The SMM is triggered by writing to the special ioports ``0xb2`` and ``0x84``,
-and may cause short hangs when the BIOS code is taking too long to
+The SMM may cause short hangs when the BIOS code is taking too long to
 execute.

 The SMM handler inside the system BIOS looks at the contents of the
@@ -210,7 +209,40 @@ The SMM handler can signal a failure by either:

 - setting the lower sixteen bits of ``eax`` to ``0xffff``
 - not modifying ``eax`` at all
-- setting the carry flag
+- setting the carry flag (legacy SMM interface only)
+
+Legacy SMM Interface
+--------------------
+
+When using the legacy SMM interface, a SMM is triggered by writing the least significant byte
+of the command code to the special ioports ``0xb2`` and ``0x84``. This interface is not
+described inside the ACPI tables and can thus only be detected by issuing a test SMM call.
+
+WMI SMM Interface
+-----------------
+
+On modern Dell machines, the SMM calls are done over ACPI WMI:
+
+::
+
+ #pragma namespace("\\\\.\\root\\dcim\\sysman\\diagnostics")
+ [WMI, Provider("Provider_DiagnosticsServices"), Dynamic, Locale("MS\\0x409"),
+  Description("RunDellDiag"), guid("{F1DDEE52-063C-4784-A11E-8A06684B9B01}")]
+ class LegacyDiags {
+  [key, read] string InstanceName;
+  [read] boolean Active;
+
+  [WmiMethodId(1), Implemented, read, write, Description("Legacy Method ")]
+  void Execute([in, out] uint32 EaxLen, [in, out, WmiSizeIs("EaxLen") : ToInstance] uint8 EaxVal[],
+               [in, out] uint32 EbxLen, [in, out, WmiSizeIs("EbxLen") : ToInstance] uint8 EbxVal[],
+               [in, out] uint32 EcxLen, [in, out, WmiSizeIs("EcxLen") : ToInstance] uint8 EcxVal[],
+               [in, out] uint32 EdxLen, [in, out, WmiSizeIs("EdxLen") : ToInstance] uint8 EdxVal[]);
+ };
+
+Some machines support only the WMI SMM interface, while some machines support both interfaces.
+The driver automatically detects which interfaces are present and will use the WMI SMM interface
+if the legacy SMM interface is not present. The WMI SMM interface is usually slower than the
+legacy SMM interface since ACPI methods need to be called in order to trigger a SMM.

 SMM command codes
 -----------------
--
2.39.2