Re: [KVM TSC trapping / migration 1/2] Add TSC trapping for SVM andVMX

[Zachary Amsden]

On 01/07/2011 01:23 AM, Marcelo Tosatti wrote:
> 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@xxxxxxxxxx>
> > ---
> >   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.
>    

Yes, will do.

> - KVM_TSC_CONTROL ioctl ignores flags field.
>    

Oops.

> - What is the purpose of PVCLOCK_TSC_TRAPPED_BIT?
>    

To allow RDTSCP optimizations for KVM clock when TSC is trapped (because 
a userspace application requires strict TSC).

> - Fail to see purpose of module parameters. Configuration from qemu
>    should be enough?
>    

For users with older versions of qemu who wish to take advantage of the 
feature, or performance / bug testing.  And oops here, the tsc_trap 
should not default to on.
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