KVM: x86: Document an erratum in KVM_SET_VCPU_EVENTS on Intel CPUs

Document a flaw in KVM's ABI which lets userspace attempt to inject a
"bad" hardware exception event, and thus induce VM-Fail on Intel CPUs.
Fixing the flaw is a fool's errand, as AMD doesn't sanity check the
validity of the error code, Intel CPUs that support CET relax the check
for Protected Mode, userspace can change the mode after queueing an
exception, KVM ignores the error code when emulating Real Mode exceptions,
and so on and so forth.

The VM-Fail itself doesn't harm KVM or the kernel beyond triggering a
ratelimited pr_warn(), so just document the oddity.

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>

Documentation/virt/kvm/x86/errata.rst

···
 to be present likely predates these CPUID feature bits, and therefore
 doesn't know to check for them anyway.
 
+``KVM_SET_VCPU_EVENTS`` issue
+-----------------------------
+
+Invalid KVM_SET_VCPU_EVENTS input with respect to error codes *may* result in
+failed VM-Entry on Intel CPUs.  Pre-CET Intel CPUs require that exception
+injection through the VMCS correctly set the "error code valid" flag, e.g.
+require the flag be set when injecting a #GP, clear when injecting a #UD,
+clear when injecting a soft exception, etc.  Intel CPUs that enumerate
+IA32_VMX_BASIC[56] as '1' relax VMX's consistency checks, and AMD CPUs have no
+restrictions whatsoever.  KVM_SET_VCPU_EVENTS doesn't sanity check the vector
+versus "has_error_code", i.e. KVM's ABI follows AMD behavior.
+
 Nested virtualization features
 ------------------------------