doc: add boot protocol page for cpu templates

Signed-off-by: Nikita Kalyazin <[email protected]>

Author: kalyazin

docs/cpu_templates/boot-protocol.md

@@ -0,0 +1,33 @@
+# Boot protocol register settings
+
+Firecracker makes certain modifications to the guest's registers
+regardless of whether a CPU template is used to comply with the boot protocol.
+If a CPU template is used the boot protocol settings are performed _after_ the
+CPU template is applied. That means that if the CPU template configures CPUID
+bits used in the boot protocol settings, they will be overwritten.
+
+See also: [CPUID normalization](cpuid-normalization.md)
+
+## Boot protocol MSRs (x86_64 only)
+
+On x86_64, the following MSRs are set to `0`:
+
+- MSR_IA32_SYSENTER_CS
+- MSR_IA32_SYSENTER_ESP
+- MSR_IA32_SYSENTER_EIP
+- MSR_STAR
+- MSR_CSTAR
+- MSR_KERNEL_GS_BASE
+- MSR_SYSCALL_MASK
+- MSR_LSTAR
+- MSR_IA32_TSC
+
+and MSR_IA32_MISC_ENABLE is set to `1`.
+
+## Boot protocol ARM registers (aarch64 only)
+
+On aarch64, the following registers are set:
+
+- PSTATE to PSR_MODE_EL1h | PSR_A_BIT | PSR_F_BIT | PSR_I_BIT | PSR_D_BIT
+- PC to kernel load address (vCPU0 only)
+- X0 to DTB/FDT address (vCPU0 only)