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Subject[RFC 0/6] vDSO support for Hyper-V guest on ARM64

This is the RFC patchset for vDSO support in ARM64 Hyper-V guest. To
test it, Michael's ARM64 support patchset:

is needed.

Similar as x86, Hyper-V on ARM64 use a TSC page for guests to read
the virtualized hardware timer, this TSC page is read-only for the
guests, so could be used for vDSO data page. And the vDSO (userspace)
code could use the same code for timer reading as kernel, since
they read the same TSC page.

This patchset therefore extends ARM64's __vsdo_init() to allow multiple
data pages and introduces the vclock_mode concept similar to x86 to
allow different platforms (bare-metal, Hyper-V, etc.) to switch to
different __arch_get_hw_counter() implementations. The rest of this
patchset does the necessary setup for Hyper-V guests: mapping tsc page,
enabling userspace to read cntvct, etc. to enable vDSO.

This patchset consists of 6 patches:

patch #1 allows hv_get_raw_timer() definition to be overridden for
userspace and kernel to share the same hv_read_tsc_page() definition.

patch #2 extends ARM64 to support multiple vDSO data pages.

patch #3 introduces vclock_mode similiar to x86 to allow different
__arch_get_hw_counter() implementations for different clocksources.

patch #4 maps Hyper-V TSC page into vDSO data page.

patch #5 allows userspace to read cntvct, so that userspace can
efficiently read the clocksource.

patch #6 enables the vDSO for ARM64 Hyper-V guest.

The whole patchset is based on v5.5-rc1 plus Michael's ARM64 support
patchset, and I've done a few tests with:

Comments and suggestions are welcome!


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