Takahiro Itazuri <itazur@xxxxxxxxxx> writes: > Similar to the sysfs files for mds and tsx_async_abort, that for > mmio_stale_data also provides the best effort mitigation mode which > invokes the mitigation instructions without a guarantee they clear the > CPU buffers to address virtualized scenarios. Adds description for the > mode in the mmio_stale_data's page to make it self-contained. > > Signed-off-by: Takahiro Itazuri <itazur@xxxxxxxxxx> > --- > .../hw-vuln/processor_mmio_stale_data.rst | 19 ++++++++++++++++++- > 1 file changed, 18 insertions(+), 1 deletion(-) > > 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..c1c96eeb8 100644 > --- a/Documentation/admin-guide/hw-vuln/processor_mmio_stale_data.rst > +++ b/Documentation/admin-guide/hw-vuln/processor_mmio_stale_data.rst > @@ -226,7 +226,7 @@ The possible values in this file are: > - 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. > + mitigation is enabled on a best effort basis. See :ref:`best_effort`. > * - 'Mitigation: Clear CPU buffers' > - The processor is vulnerable and the CPU buffer clearing mitigation is > enabled. > @@ -254,6 +254,23 @@ the above information: > 'SMT Host state unknown' Kernel runs in a VM, Host SMT state unknown > ======================== =========================================== > > +.. _best_effort: > + > +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 mmio_stale_data sysfs file reflects this situation accordingly. > + So the change seems OK, but why not just describe the "best effort" mode in the place where you refer to it, rather than creating a forward reference? Thanks, jon
