lkml.org 
[lkml]   [2011]   [Jan]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [KVM TSC trapping / migration 1/2] Add TSC trapping for SVM and VMX
    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@redhat.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.
    >

    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.


    \
     
     \ /
      Last update: 2011-01-09 09:09    [W:0.024 / U:61.876 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site