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

Zachary Amsden
2011-01-06T20:03:52Z

On 01/06/2011 01:32 AM, Avi Kivity wrote:
> On 01/06/2011 12:10 PM, 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(-)
>
>
> Haven't reviewed yet, but Documentation/kvm/api.txt is missing here.
>

That will be included when I port to upstream head. When dealing with
software documentation, too much is never enough.

Zach