Re: [PATCH] KVM: VMX: remove LFENCE in vmx_spec_ctrl_restore_host()

[Josh Poimboeuf]

On Tue, Jun 06, 2023 at 01:20:52AM +0100, Andrew Cooper wrote:

<clip very useful summary which belongs in git somewhere>

> But, the safety of vmx_spec_ctrl_restore_host() in the first place
> depends on the early return never ever becoming a conditional,

Good point.  And that would be easier to overlook in C.

> and the compiler never emitting a call to memcpy()/memset()/whatever
> behind your back - something which is not prohibited by noinstr.

Au contraire, objtool has checking for that:

	if (state->noinstr && state->instr <= 0 &&
	    !noinstr_call_dest(file, insn, insn_call_dest(insn))) {
		WARN_INSN(insn, "call to %s() leaves .noinstr.text section", call_dest_name(insn));
		return 1;
	}

Regardless, despite being the person who wrote this thing in C to begin
with, I believe asm really is a better fit due to the delicate and
precise nature of the mitigations.

-- 
Josh