代码在这里 arch/x86/kernel/cpu/microcode/

https://superuser.com/questions/934752/do-arm-processors-like-cortex-a9-use-microcode

https://wiki.gentoo.org/wiki/AMD_microcode

dracut 这个警告有意思

dracut --force --kver $(uname -r)
dracut: Disabling early microcode, because kernel does not support it. CONFIG_MICROCODE_[AMD|INTEL]!=y

看看

svm_check_emulate_instruction

	/*
	 * Detect and workaround Errata 1096 Fam_17h_00_0Fh.
	 *
	 * Errata:
	 * When CPU raises #NPF on guest data access and vCPU CR4.SMAP=1, it is
	 * possible that CPU microcode implementing DecodeAssist will fail to
	 * read guest memory at CS:RIP and vmcb.GuestIntrBytes will incorrectly
	 * be '0'.  This happens because microcode reads CS:RIP using a _data_
	 * loap uop with CPL=0 privileges.  If the load hits a SMAP #PF, ucode
	 * gives up and does not fill the instruction bytes buffer.
	 *
	 * As above, KVM reaches this point iff the VM is an SEV guest, the CPU
	 * supports DecodeAssist, a #NPF was raised, KVM's page fault handler
	 * triggered emulation (e.g. for MMIO), and the CPU returned 0 in the
	 * GuestIntrBytes field of the VMCB.
	 *
	 * This does _not_ mean that the erratum has been encountered, as the
	 * DecodeAssist will also fail if the load for CS:RIP hits a legitimate
	 * #PF, e.g. if the guest attempt to execute from emulated MMIO and
	 * encountered a reserved/not-present #PF.
	 *
	 * To hit the erratum, the following conditions must be true:
	 *    1. CR4.SMAP=1 (obviously).
	 *    2. CR4.SMEP=0 || CPL=3.  If SMEP=1 and CPL<3, the erratum cannot
	 *       have been hit as the guest would have encountered a SMEP
	 *       violation #PF, not a #NPF.
	 *    3. The #NPF is not due to a code fetch, in which case failure to
	 *       retrieve the instruction bytes is legitimate (see abvoe).
	 *
	 * In addition, don't apply the erratum workaround if the #NPF occurred
	 * while translating guest page tables (see below).
	 */

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