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commit 5ce344beaca688f4cdea07045e0b8f03dc537e74 upstream. When done from a virtual machine, instructions that touch APIC memory must be emulated. By convention, MMIO accesses are typically performed via io.h helpers such as readl() or writeq() to simplify instruction emulation/decoding (ex: in KVM hosts and SEV guests) [0]. Currently, native_apic_mem_read() does not follow this convention, allowing the compiler to emit instructions other than the MOV instruction generated by readl(). In particular, when the kernel is compiled with clang and run as a SEV-ES or SEV-SNP guest, the compiler would emit a TESTL instruction which is not supported by the SEV-ES emulator, causing a boot failure in that environment. It is likely the same problem would happen in a TDX guest as that uses the same instruction emulator as SEV-ES. To make sure all emulators can emulate APIC memory reads via MOV, use the readl() function in native_apic_mem_read(). It is expected that any emulator would support MOV in any addressing mode as it is the most generic and is what is usually emitted currently. The TESTL instruction is emitted when native_apic_mem_read() is inlined into apic_mem_wait_icr_idle(). The emulator comes from insn_decode_mmio() in arch/x86/lib/insn-eval.c. It's not worth it to extend insn_decode_mmio() to support more instructions since, in theory, the compiler could choose to output nearly any instruction for such reads which would bloat the emulator beyond reason. [0] https://lore.kernel.org/all/20220405232939.73860-12-kirill.shutemov@linux.intel.com/ [ bp: Massage commit message, fix typos. ] Signed-off-by: Adam Dunlap <acdunlap@google.com> Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de> Reviewed-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Ard Biesheuvel <ardb@kernel.org> Tested-by: Kevin Loughlin <kevinloughlin@google.com> Cc: <stable@vger.kernel.org> Link: https://lore.kernel.org/r/20240318230927.2191933-1-acdunlap@google.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
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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.