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

[Boqun Feng]

Hi Vincenzo,

On Thu, Jan 23, 2020 at 10:48:07AM +0000, Vincenzo Frascino wrote:
> Hi Boqun Feng,
> 
> sorry for the late reply.
> 

That's OK, thanks for your review ;-)

> On 16/12/2019 00:19, Boqun Feng wrote:
> > Hi,
> > 
> > This is the RFC patchset for vDSO support in ARM64 Hyper-V guest. To
> > test it, Michael's ARM64 support patchset:
> > 
> > 	https://lore.kernel.org/linux-arm-kernel/1570129355-16005-1-git-send-email-mikelley@xxxxxxxxxxxxx/
> > 
> > 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.
> > 
> 
> I had a look to your patches and overall, I could not understand why we can't
> use the arch_timer to do the same things you are doing with the one you
> introduced in this series. What confuses me is that KVM works just fine with the
> arch_timer which was designed with virtualization in mind. Why do we need
> another one? Could you please explain?
> 

Please note that the guest VM on Hyper-V for ARM64 doesn't use
arch_timer as the clocksource. See:

	https://lore.kernel.org/linux-arm-kernel/1570129355-16005-7-git-send-email-mikelley@xxxxxxxxxxxxx/

,  ACPI_SIG_GTDT is used for setting up Hyper-V synthetic clocksource
and other initialization work.

So just to be clear, your suggestion is

1) Hyper-V guest on ARM64 should use arch_timer as clocksource and vDSO
will just work.

or

2) Even though arch_timer is not used as the clocksource, we can still
use it for vDSO.

?

Regards,
Boqun

> > 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:
> > 
> > 	https://github.com/nlynch-mentor/vdsotest
> > 
> > Comments and suggestions are welcome!
> > 
> > Regards,
> > Boqun
> > 
> > _______________________________________________
> > linux-arm-kernel mailing list
> > linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> > 
> 
> -- 
> Regards,
> Vincenzo