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Message-ID: <20230831111847.71030-1-itazur@amazon.com>
Date: Thu, 31 Aug 2023 12:18:47 +0100
From: Takahiro Itazuri <itazur@...zon.com>
To: <linux-kernel@...r.kernel.org>, <linux-doc@...r.kernel.org>
CC: Jonathan Corbet <corbet@....net>,
Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
Josh Poimboeuf <jpoimboe@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Borislav Petkov <bp@...en8.de>,
"Thomas Gleixner" <tglx@...utronix.de>,
Takahiro Itazuri <zulinx86@...il.com>,
Takahiro Itazuri <itazur@...zon.com>
Subject: [PATCH v2] docs: Update desc of best effort mode
Moves the description of the best effort mitigation mode to the table of
the possible values in the mds and tsx_async_abort docs, and adds the
same one to the mmio_stale_data doc.
Signed-off-by: Takahiro Itazuri <itazur@...zon.com>
---
v1 -> v2: https://lore.kernel.org/all/20230830144426.80258-1-itazur@amazon.com/
- Puts the desc into the table of the possible values.
---
Documentation/admin-guide/hw-vuln/mds.rst | 33 ++++++++-----------
.../hw-vuln/processor_mmio_stale_data.rst | 13 +++++++-
.../admin-guide/hw-vuln/tsx_async_abort.rst | 32 ++++++++----------
3 files changed, 38 insertions(+), 40 deletions(-)
diff --git a/Documentation/admin-guide/hw-vuln/mds.rst b/Documentation/admin-guide/hw-vuln/mds.rst
index 48ca0bd85..0fe98151a 100644
--- a/Documentation/admin-guide/hw-vuln/mds.rst
+++ b/Documentation/admin-guide/hw-vuln/mds.rst
@@ -102,9 +102,19 @@ The possible values in this file are:
* - 'Vulnerable'
- The processor is vulnerable, but no mitigation enabled
* - 'Vulnerable: Clear CPU buffers attempted, no microcode'
- - The processor is vulnerable but microcode is not updated.
-
- The mitigation is enabled on a best effort basis. See :ref:`vmwerv`
+ - The processor is vulnerable but microcode is not updated. The
+ mitigation is enabled on a best effort basis.
+
+ If the processor is vulnerable but the availability of the microcode
+ based mitigation mechanism is not advertised via CPUID, the kernel
+ selects a best effort mitigation mode. This mode invokes the mitigation
+ instructions without a guarantee that they clear the CPU buffers.
+
+ This is done to address virtualization scenarios where the host has the
+ microcode update applied, but the hypervisor is not yet updated to
+ expose the CPUID to the guest. If the host has updated microcode the
+ protection takes effect; otherwise a few CPU cycles are wasted
+ pointlessly.
* - 'Mitigation: Clear CPU buffers'
- The processor is vulnerable and the CPU buffer clearing mitigation is
enabled.
@@ -119,23 +129,6 @@ to the above information:
'SMT Host state unknown' Kernel runs in a VM, Host SMT state unknown
======================== ============================================
-.. _vmwerv:
-
-Best effort mitigation mode
-^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
- If the processor is vulnerable, but the availability of the microcode based
- mitigation mechanism is not advertised via CPUID the kernel selects a best
- effort mitigation mode. This mode invokes the mitigation instructions
- without a guarantee that they clear the CPU buffers.
-
- This is done to address virtualization scenarios where the host has the
- microcode update applied, but the hypervisor is not yet updated to expose
- the CPUID to the guest. If the host has updated microcode the protection
- takes effect otherwise a few cpu cycles are wasted pointlessly.
-
- The state in the mds sysfs file reflects this situation accordingly.
-
Mitigation mechanism
-------------------------
diff --git a/Documentation/admin-guide/hw-vuln/processor_mmio_stale_data.rst b/Documentation/admin-guide/hw-vuln/processor_mmio_stale_data.rst
index c98fd1190..1302fd1b5 100644
--- a/Documentation/admin-guide/hw-vuln/processor_mmio_stale_data.rst
+++ b/Documentation/admin-guide/hw-vuln/processor_mmio_stale_data.rst
@@ -225,8 +225,19 @@ The possible values in this file are:
* - 'Vulnerable'
- The processor is vulnerable, but no mitigation enabled
* - 'Vulnerable: Clear CPU buffers attempted, no microcode'
- - The processor is vulnerable, but microcode is not updated. The
+ - The processor is vulnerable but microcode is not updated. The
mitigation is enabled on a best effort basis.
+
+ If the processor is vulnerable but the availability of the microcode
+ based mitigation mechanism is not advertised via CPUID, the kernel
+ selects a best effort mitigation mode. This mode invokes the mitigation
+ instructions without a guarantee that they clear the CPU buffers.
+
+ This is done to address virtualization scenarios where the host has the
+ microcode update applied, but the hypervisor is not yet updated to
+ expose the CPUID to the guest. If the host has updated microcode the
+ protection takes effect; otherwise a few CPU cycles are wasted
+ pointlessly.
* - 'Mitigation: Clear CPU buffers'
- The processor is vulnerable and the CPU buffer clearing mitigation is
enabled.
diff --git a/Documentation/admin-guide/hw-vuln/tsx_async_abort.rst b/Documentation/admin-guide/hw-vuln/tsx_async_abort.rst
index 014167ef8..c6400fe6b 100644
--- a/Documentation/admin-guide/hw-vuln/tsx_async_abort.rst
+++ b/Documentation/admin-guide/hw-vuln/tsx_async_abort.rst
@@ -98,7 +98,19 @@ The possible values in this file are:
* - 'Vulnerable'
- The CPU is affected by this vulnerability and the microcode and kernel mitigation are not applied.
* - 'Vulnerable: Clear CPU buffers attempted, no microcode'
- - The system tries to clear the buffers but the microcode might not support the operation.
+ - The processor is vulnerable but microcode is not updated. The
+ mitigation is enabled on a best effort basis.
+
+ If the processor is vulnerable but the availability of the microcode
+ based mitigation mechanism is not advertised via CPUID, the kernel
+ selects a best effort mitigation mode. This mode invokes the mitigation
+ instructions without a guarantee that they clear the CPU buffers.
+
+ This is done to address virtualization scenarios where the host has the
+ microcode update applied, but the hypervisor is not yet updated to
+ expose the CPUID to the guest. If the host has updated microcode the
+ protection takes effect; otherwise a few CPU cycles are wasted
+ pointlessly.
* - 'Mitigation: Clear CPU buffers'
- The microcode has been updated to clear the buffers. TSX is still enabled.
* - 'Mitigation: TSX disabled'
@@ -106,24 +118,6 @@ The possible values in this file are:
* - 'Not affected'
- The CPU is not affected by this issue.
-.. _ucode_needed:
-
-Best effort mitigation mode
-^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-If the processor is vulnerable, but the availability of the microcode-based
-mitigation mechanism is not advertised via CPUID the kernel selects a best
-effort mitigation mode. This mode invokes the mitigation instructions
-without a guarantee that they clear the CPU buffers.
-
-This is done to address virtualization scenarios where the host has the
-microcode update applied, but the hypervisor is not yet updated to expose the
-CPUID to the guest. If the host has updated microcode the protection takes
-effect; otherwise a few CPU cycles are wasted pointlessly.
-
-The state in the tsx_async_abort sysfs file reflects this situation
-accordingly.
-
Mitigation mechanism
--------------------
--
2.40.1
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