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

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Date:   Fri, 16 Feb 2018 12:04:36 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>, tglx@...utronix.de,
        x86@...nel.org, kvm@...r.kernel.org, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, arjan.van.de.ven@...el.com,
        dave.hansen@...el.com
Cc:     Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future
 CPUs

On 16/02/2018 11:21, David Woodhouse wrote:
> Why? With IBRS_ALL the guest *never* gets to affect the actual hardware
> MSR, which is always on. The MSR is purely an emulated no-op. Why does
> that affect migration?

Because even if the host has IBRS_ALL, as long as you want to migrate to
a system without IBRS_ALL the guest will likely not have it.  You can
fake IBRS_ALL on the older system after migration, and forcing the guest
to always run with IBRS=1 even when in user mode; that is slow.  Or...

> Even if the guest doesn't have/support IBRS_ALL, and is frobbing the
> (now emulated) MSR on every kernel entry/exit, that's *still* going to
> be a metric shitload faster than what it *thought* it was doing.

... you are making every kernel entry/exit 3 times slower by adding two
KVM exits (both hypervisor traps and syscalls are in the 1000-1500 clock
cycles ballpark).  That cannot be fast at all.

Paolo