From: Paolo Bonzini <pbonzini@redhat.com>
To: David Woodhouse <dwmw2@infradead.org>,
tglx@linutronix.de, x86@kernel.org, kvm@vger.kernel.org,
torvalds@linux-foundation.org, linux-kernel@vger.kernel.org,
arjan.van.de.ven@intel.com, dave.hansen@intel.com
Cc: Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future CPUs
Date: Fri, 16 Feb 2018 12:04:36 +0100 [thread overview]
Message-ID: <d61edc94-8a51-62bd-65ed-8068451ca5b5@redhat.com> (raw)
In-Reply-To: <1518776517.7876.21.camel@infradead.org>
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
next prev parent reply other threads:[~2018-02-16 11:04 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-12 15:27 [PATCH 1/2] x86/speculation: Correct Speculation Control microcode blacklist again David Woodhouse
2018-02-12 15:27 ` [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future CPUs David Woodhouse
2018-02-13 7:47 ` Ingo Molnar
2018-02-13 8:12 ` David Woodhouse
2018-02-13 8:02 ` Paolo Bonzini
2018-02-13 8:15 ` David Woodhouse
2018-02-13 9:58 ` Paolo Bonzini
2018-02-13 10:21 ` David Woodhouse
2018-02-13 10:36 ` David Woodhouse
2018-02-13 10:41 ` Paolo Bonzini
2018-02-13 10:53 ` David Woodhouse
2018-02-13 10:55 ` Paolo Bonzini
2018-02-16 9:58 ` David Woodhouse
2018-02-16 10:08 ` Paolo Bonzini
2018-02-16 10:21 ` David Woodhouse
2018-02-16 11:04 ` Paolo Bonzini [this message]
2018-02-16 12:10 ` David Woodhouse
2018-02-19 23:37 ` Jon Masters
2018-02-19 23:42 ` Van De Ven, Arjan
2018-02-19 23:53 ` valdis.kletnieks
2018-02-20 0:00 ` Van De Ven, Arjan
2018-02-20 0:13 ` Alan Cox
2018-02-20 0:43 ` Linus Torvalds
2018-02-20 1:03 ` Alan Cox
2018-02-20 1:08 ` Van De Ven, Arjan
2018-02-20 8:52 ` Thomas Gleixner
2018-02-20 11:43 ` Paolo Bonzini
2018-02-20 14:08 ` Van De Ven, Arjan
2018-02-20 14:46 ` Paolo Bonzini
2018-02-20 14:59 ` Van De Ven, Arjan
2018-02-20 15:09 ` Paolo Bonzini
2018-02-23 18:12 ` Is: RSB Alternative bit in IA32_ARCH_CAPABILITIES Was:Re: " Konrad Rzeszutek Wilk
2018-02-23 18:18 ` Van De Ven, Arjan
2018-02-15 15:21 ` Pavel Machek
2018-02-13 8:57 ` [tip:x86/pti] x86/speculation: Correct Speculation Control microcode blacklist again tip-bot for David Woodhouse
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