v4:
  * The refactor Andrew asked for to avoid a conditional check on
    early_microcode_init(). I'm not convinced it's clearer, but it's not
    much more complicated either, so I don't mind.
  * Removed microcode_ops builders in favour of a separate Intel-specific
    function to check whether the system can load new microcode. This keeps
    the static structs in the right sections.

Under certain conditions a CPU may not be able to perform microcode updates
even if hardware exists to that effect. In particular:

 * If Xen runs under certain hypervisors they won't allow microcode
   updates, and will signal this fact by reporting a microcode revision of
   -1.
 * If the DIS_MCU_LOAD bit is set, which is expected in some baremetal
   clouds where the owner may not trust the tenant, then the CPU is not
   capable of loading new microcode.

This series adds logic so that in both of these cases we don't needlessly
attempt updates that are not going to succeed. Patch summary:

Patch 1 Introduces the logic to print the microcode revision if at all
        possible

Patch 2 Ignores microcode facilities when the current microcode revision is -1

Patch 3 Moves the MSR_ARCH_CAPS read in tsx_init() to early_cpu_init() and
        early_microcode_init()

Patch 4 Adds the logic to detect microcode updates being disabled on Intel.

Alejandro Vallejo (4):
  x86/microcode: Allow reading microcode revision even if it can't be
    updated
  x86/microcode: Ignore microcode loading interface for revision = -1
  x86: Read MSR_ARCH_CAPS immediately after early_microcode_init()
  x86/microcode: Disable microcode update handler if DIS_MCU_UPDATE is
    set

 xen/arch/x86/cpu/common.c             |  5 ++++
 xen/arch/x86/cpu/microcode/core.c     | 41 +++++++++++++++++++++++----
 xen/arch/x86/cpu/microcode/intel.c    | 13 +++++++++
 xen/arch/x86/cpu/microcode/private.h  |  7 +++++
 xen/arch/x86/include/asm/cpufeature.h |  1 +
 xen/arch/x86/include/asm/msr-index.h  |  5 ++++
 xen/arch/x86/tsx.c                    | 16 +++--------
 7 files changed, 70 insertions(+), 18 deletions(-)

-- 
2.34.1