linux-kernel - Re: [PATCH v3 2/4] x86/speculation: Support "Enhanced IBRS" on future CPUs

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Date:   Tue, 20 Feb 2018 12:26:38 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Woodhouse <dwmw@...zon.co.uk>, tglx@...utronix.de,
        karahmed@...zon.de, x86@...nel.org, kvm@...r.kernel.org,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        bp@...en8.de, peterz@...radead.org, jmattson@...gle.com,
        rkrcmar@...hat.com, arjan.van.de.ven@...el.com,
        dave.hansen@...el.com, mingo@...nel.org
Subject: Re: [PATCH v3 2/4] x86/speculation: Support "Enhanced IBRS" on future
 CPUs

On 19/02/2018 11:50, David Woodhouse wrote:
> Cope with this by trapping and emulating *all* access to SPEC_CTRL from
> KVM guests when the IBRS_ALL feature is present, so it can never be
> turned off. Guests who see IBRS_ALL should never do anything except
> turn it on at boot anyway. And if they didn't know about IBRS_ALL and
> they keep frobbing IBRS on every kernel entry/exit... well the vmexit
> for a no-op is probably going to be faster than they were expecting
> anyway, so they'll live.

The problem is, it isn't.  On a Haswell (which has fairly slow
SPEC_CTRL) toggling IBRS is 200 cycles.  This gives a context switch
time of around 2000 clock cycles with PTI enabled.

This is fairly awful, but with a vmexit cost of ~1100 cycles that goes
up to 2000+(1100-200)*2 = 3800.  That's more or less doubling the cost
of a system call.

With newer machines SPEC_CTRL cost goes down but vmexit cost doesn't, so
it's only worse.

For now, we really should do something like

	if (vmx->spec_ctrl != host_spec_ctrl)
		wrmsrl(MSR_IA32_SPEC_CTRL, host_spec_ctrl);
	else
		lfence();

which later can become

	if (vmx->spec_ctrl != host_spec_ctrl)
		wrmsrl(MSR_IA32_SPEC_CTRL, host_spec_ctrl);
	else {
		/* lfence not needed if host_spec_ctrl == 0 */
		if (static_cpu_has(BUG_REALLY_WANTS_IBRS))
			nospec_barrier();
	}

Paolo