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

stable inclusion
from stable-v5.10.173
commit 511e27e5fdd658e6cb06b4947fb0d3ac76163776
category: feature
bugzilla: https://gitee.com/openeuler/intel-kernel/issues/I873BU
CVE: N/A
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=511e27e5fdd658e6cb06b4947fb0d3ac76163776



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Intel-SIG: commit 511e27e5fdd6 x86/microcode: Check CPU capabilities after late microcode update correctly.
Backport x86/microcode related patches from 5.10.173 upstream.

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[ Upstream commit c0dd9245 ]

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:

  									     ^^^

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: 1008c52c ("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


Signed-off-by: Sasha Levin <sashal@kernel.org>
[ Aichun Shi: amend commit log ]
Signed-off-by: Aichun Shi <aichun.shi@intel.com>