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When running virtualized, MTRR access can be reduced (e.g. in Xen PV guests or when running as a SEV-SNP guest under Hyper-V). Typically, the hypervisor will not advertize the MTRR feature in CPUID data, resulting in no MTRR memory type information being available for the kernel. This has turned out to result in problems (Link tags below): - Hyper-V SEV-SNP guests using uncached mappings where they shouldn't - Xen PV dom0 mapping memory as WB which should be UC- instead Solve those problems by allowing an MTRR static state override, overwriting the empty state used today. In case such a state has been set, don't call get_mtrr_state() in mtrr_bp_init(). The set state will only be used by mtrr_type_lookup(), as in all other cases mtrr_enabled() is being checked, which will return false. Accept the overwrite call only for selected cases when running as a guest. Disable X86_FEATURE_MTRR in order to avoid any MTRR modifications by just refusing them. [ bp: Massage. ] Signed-off-by: Juergen Gross <jgross@suse.com> Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de> Tested-by: Michael Kelley <mikelley@microsoft.com> Link: https://lore.kernel.org/all/4fe9541e-4d4c-2b2a-f8c8-2d34a7284930@nerdbynature.de/ Link: https://lore.kernel.org/lkml/BYAPR21MB16883ABC186566BD4D2A1451D7FE9@BYAPR21MB1688.namprd21.prod.outlook.com Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
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.