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Date:   Mon, 5 Jun 2023 20:59:53 -0700
From:   Josh Poimboeuf <jpoimboe@kernel.org>
To:     Andrew Cooper <andrew.cooper3@citrix.com>
Cc:     Jon Kohler <jon@nutanix.com>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Daniel Sneddon <daniel.sneddon@linux.intel.com>,
        "kvm @ vger . kernel . org" <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] KVM: VMX: remove LFENCE in vmx_spec_ctrl_restore_host()
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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