x86/microcode: Check CPU capabilities after late microcode update correctly

x86/microcode: Check CPU capabilities after late microcode update correctly

The kernel caches each CPU's feature bits at boot in an x86_capability[]
structure. However, the capabilities in the BSP's copy can be turned off
as a result of certain command line parameters or configuration
restrictions, for example the SGX bit. This can cause a mismatch when
comparing the values before and after the microcode update.

Another example is X86_FEATURE_SRBDS_CTRL which gets added only after
microcode update:

  --- cpuid.before	2023-01-21 14:54:15.652000747 +0100
  +++ cpuid.after	2023-01-21 14:54:26.632001024 +0100
  @@ -10,7 +10,7 @@ CPU:
      0x00000004 0x04: eax=0x00000000 ebx=0x00000000 ecx=0x00000000 edx=0x00000000
      0x00000005 0x00: eax=0x00000040 ebx=0x00000040 ecx=0x00000003 edx=0x11142120
      0x00000006 0x00: eax=0x000027f7 ebx=0x00000002 ecx=0x00000001 edx=0x00000000
  -   0x00000007 0x00: eax=0x00000000 ebx=0x029c6fbf ecx=0x40000000 edx=0xbc002400
  +   0x00000007 0x00: eax=0x00000000 ebx=0x029c6fbf ecx=0x40000000 edx=0xbc002e00
  									     ^^^

and which proves for a gazillionth time that late loading is a bad bad
idea.

microcode_check() is called after an update to report any previously
cached CPUID bits which might have changed due to the update.

Therefore, store the cached CPU caps before the update and compare them
with the CPU caps after the microcode update has succeeded.

Thus, the comparison is done between the CPUID *hardware* bits before
and after the upgrade instead of using the cached, possibly runtime
modified values in BSP's boot_cpu_data copy.

As a result, false warnings about CPUID bits changes are avoided.

  [ bp:
  	- Massage.
	- Add SRBDS_CTRL example.
	- Add kernel-doc.
	- Incorporate forgotten review feedback from dhansen.
	]

Fixes: 1008c52c09dc ("x86/CPU: Add a microcode loader callback")
Signed-off-by: Ashok Raj <ashok.raj@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230109153555.4986-3-ashok.raj@intel.com