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

Zachary Amsden
2011-01-09T08:05:59Z

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.