661b1c6169
If the kernel uses a 2MB or larger directmap mapping to write to an address, and that mapping contains any 4KB pages that are set to private in the RMP table, an RMP #PF will trigger and cause a host crash. SNP-aware code that owns the private PFNs will never attempt such a write, but other kernel tasks writing to other PFNs in the range may trigger these checks inadvertently due to writing to those other PFNs via a large directmap mapping that happens to also map a private PFN. Prevent this by splitting any 2MB+ mappings that might end up containing a mix of private/shared PFNs as a result of a subsequent RMPUPDATE for the PFN/rmp_level passed in. Another way to handle this would be to limit the directmap to 4K mappings in the case of hosts that support SNP, but there is potential risk for performance regressions of certain host workloads. Handling it as-needed results in the directmap being slowly split over time, which lessens the risk of a performance regression since the more the directmap gets split as a result of running SNP guests, the more likely the host is being used primarily to run SNP guests, where a mostly-split directmap is actually beneficial since there is less chance of TLB flushing and cpa_lock contention being needed to perform these splits. Cases where a host knows in advance it wants to primarily run SNP guests and wishes to pre-split the directmap can be handled by adding a tuneable in the future, but preliminary testing has shown this to not provide a signficant benefit in the common case of guests that are backed primarily by 2MB THPs, so it does not seem to be warranted currently and can be added later if a need arises in the future. Signed-off-by: Michael Roth <michael.roth@amd.com> Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de> Acked-by: Vlastimil Babka <vbabka@suse.cz> Link: https://lore.kernel.org/r/20240126041126.1927228-12-michael.roth@amd.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.