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

[Khaled Elmously]

On 2020-08-12 12:42:42 , Kelsey Skunberg wrote:
> 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
> 
> 
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