linux-kernel - Re: [KVM TSC trapping / migration 1/2] Add TSC trapping for SVM and VMX

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Date:	Fri, 7 Jan 2011 09:23:34 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Zachary Amsden <zamsden@...hat.com>
Cc:	kvm@...r.kernel.org, Avi Kivity <avi@...hat.com>,
	Glauber Costa <glommer@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [KVM TSC trapping / migration 1/2] Add TSC trapping for SVM and
 VMX

On Thu, Jan 06, 2011 at 12:10:44AM -1000, Zachary Amsden wrote:
> Reasons to trap the TSC are numerous, but we want to avoid it as much
> as possible for performance reasons.
> 
> We provide two conservative modes via modules parameters and userspace
> hinting.  First, the module can be loaded with "tsc_auto=1" as a module
> parameter, which turns on conservative TSC trapping only when it is
> required (when unstable TSC or faster KHZ CPU is detected).
> 
> For userspace hinting, we enable trapping only if necessary.  Userspace
> can hint that a VM needs a fixed frequency TSC, and also that SMP
> stability will be required.  In that case, we conservatively turn on
> trapping when it is needed.  In addition, users may now specify the
> desired TSC rate at which to run.  If this rate differs significantly
> from the host rate, trapping will be enabled.
> 
> There is also an override control to allow TSC trapping to be turned on
> or off unconditionally for testing.
> 
> We indicate to pvclock users that the TSC is being trapped, to allow
> avoiding overhead and directly using RDTSCP (only for SVM).  This
> optimization is not yet implemented.
> 
> Signed-off-by: Zachary Amsden <zamsden@...hat.com>
> ---
>  arch/x86/include/asm/kvm_host.h    |    6 +-
>  arch/x86/include/asm/pvclock-abi.h |    1 +
>  arch/x86/kvm/svm.c                 |   20 ++++++
>  arch/x86/kvm/vmx.c                 |   21 +++++++
>  arch/x86/kvm/x86.c                 |  113 +++++++++++++++++++++++++++++++++---
>  arch/x86/kvm/x86.h                 |    2 +
>  include/linux/kvm.h                |   15 +++++
>  7 files changed, 168 insertions(+), 10 deletions(-)

- Docs / test case please.
- KVM_TSC_CONTROL ioctl ignores flags field.
- What is the purpose of PVCLOCK_TSC_TRAPPED_BIT?
- Fail to see purpose of module parameters. Configuration from qemu
  should be enough?

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