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docs/hw-vuln: 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@amazon.com>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230901082959.28310-1-itazur@amazon.com
This commit is contained in:
Takahiro Itazuri 2023-09-01 09:29:59 +01:00 committed by Jonathan Corbet
parent c9ad95adc0
commit a3c12cf3a6
3 changed files with 37 additions and 41 deletions
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32
Documentation/admin-guide/hw-vuln/mds.rst
View File

@ -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 processor is vulnerable but microcode is not updated. The
mitigation is enabled on a best effort basis.
The mitigation is enabled on a best effort basis. See :ref:`vmwerv`
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.
@ -119,24 +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
-------------------------

13
Documentation/admin-guide/hw-vuln/processor_mmio_stale_data.rst
View File

@ -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.

33
Documentation/admin-guide/hw-vuln/tsx_async_abort.rst
View File

@ -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'
@ -106,25 +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
--------------------