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Subject: Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future
 CPUs
To:     David Woodhouse <dwmw2@infradead.org>, tglx@linutronix.de,
        x86@kernel.org, kvm@vger.kernel.org, torvalds@linux-foundation.org,
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From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <d61edc94-8a51-62bd-65ed-8068451ca5b5@redhat.com>
Date:   Fri, 16 Feb 2018 12:04:36 +0100
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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