** Description changed:
+ [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.
The following two commits enable support:
9e0333ae38ee ("clocksource/drivers/hyper-v: Set TSC clocksource as default w/
InvariantTSC")
dce7cd62754b ("x86/hyperv: Allow guests to enable 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.
** Changed in: linux-azure (Ubuntu Eoan)
Status: Confirmed => Won't Fix
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https://bugs.launchpad.net/bugs/1875467
Title:
Enable Invariant TSC Support
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