[B+F:linux-azure][SRU][PATCH v2 0/4] LP#:1875467: Enable invariant TSC

[Kelsey Skunberg]

BugLink: https://bugs.launchpad.net/bugs/1875467

[Impact]

Microsoft would like to request two commits in all releases supported on
Azure. These commits benefit SAP workloads.

These requested commits will enable support for Invariant TSC in Linux
guest VMs. The invariant TSC means that the TSC continues at a fixed
rate regardless of the C-state or frequency of the processor (as long as
the processor remains in the ACPI S0 state).

These patches will allow guests to access the
HvSyntheticInvariantTscControl MSR and set bit 0 of this synthetic MSR,
which then enables the InvariantTSC feature. After setting the synthetic
MSR, CPUID will enumerate support for InvariantTSC.

[Test Case]

If the hardware supports TSC scaling, Hyper-V will set bit 15 of the
HV_PARTITION_PRIVILEGE_MASK in guest VMs with a compatible Hyper-V
configuration version. Bit 15 corresponds to the
AccessTscInvariantControls privilege. If this privilege bit is set,
guests can access the HvSyntheticInvariantTscControl MSR: guests can set
bit 0 of this synthetic MSR to enable the InvariantTSC feature.  After
setting the synthetic MSR, CPUID will enumerate support for
InvariantTSC.

[Regression Potential]

If the patches fail to enable support for Invariant TSC or doesn't set
the TSC clocksource as the default when Invariant TSC is available,
while this would be an error it wouldn't be a regression. Possibility of
finding problems using the TSC clocksource once support has been
enabled.

a 4.15 test kernel was sent to Microsoft for testing. Result came back
verified to allow enabling support for Invariant TSC and there was no
report of known regression.

The 4.15, 5.3, & 5.4 kernels with patches applied were boot tested and
verified no changes to dmesg logs relating to clocksource or tsc. tsc
autotest ran successfully on the 4.15 before and after applying the
requested patches.

We are no longer interested in applying to Eoan/linux-azure (5.3) and
only targeted to apply the patches to bionic/azure-4.15 and focal/azure.


Changes since v1: More testing was done by microsoft and myself. Results
are included in the Bug and cover letter. Also brought the Focal and
Bionic patches together into one patch series. 


Bionic/azure-4.15:

Andrea Parri (2):
  x86/hyperv: Allow guests to enable InvariantTSC
  x86/hyperv: Set TSC clocksource as default w/ InvariantTSC

 arch/x86/hyperv/hv_init.c          | 13 +++++++++++--
 arch/x86/include/asm/hyperv-tlfs.h |  5 +++++
 arch/x86/kernel/cpu/mshyperv.c     |  7 ++++++-
 3 files changed, 22 insertions(+), 3 deletions(-)


Focal/azure-5.3:

Andrea Parri (2):
  x86/hyperv: Allow guests to enable InvariantTSC
  clocksource/drivers/hyper-v: Set TSC clocksource as default w/
    InvariantTSC

 arch/x86/include/asm/hyperv-tlfs.h |  5 +++++
 arch/x86/kernel/cpu/mshyperv.c     |  7 ++++++-
 drivers/clocksource/hyperv_timer.c | 12 ++++++++++--
 3 files changed, 21 insertions(+), 3 deletions(-)
-- 
2.25.1