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

[Boqun Feng <boqun.feng@xxxxxxxxx>]

On Fri, Jan 24, 2020 at 10:24:44AM +0000, Vincenzo Frascino wrote:
> Hi Boqun Feng,
> 
> On 24/01/2020 06:32, Boqun Feng wrote:
> > Hi Vincenzo,
> > 
> 
> [...]
> 
> >>
> >> 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.
> >
> 
> I had a look a look at it and my question stands, why do we need another timer
> on arm64?
> 

Sorry for the late response. It's weekend and Chinese New Year, so I got
to spend some time making (and mostly eating) dumplings ;-)

After discussion with Michael, here is some explanation why we need
another timer:

The synthetic clocks that Hyper-V presents in a guest VM were originally
created for the x86 architecture. They provide a level of abstraction
that solves problems like continuity across live migrations where the
hardware clock (i.e., TSC in the case x86) frequency may be different
across the migration. When Hyper-V was brought to ARM64, this
abstraction was maintained to provide consistency across the x86 and
ARM64 architectures, and for both Windows and Linux guest VMs.   The
core Linux code for the Hyper-V clocks (in
drivers/clocksource/hyperv_timer.c) is architecture neutral and works on
both x86 and ARM64. As you can see, this part is done in Michael's
patchset.

Arguably, Hyper-V for ARM64 should have optimized for consistency with
the ARM64 community rather with the existing x86 implementation and
existing guest code in Windows. But at this point, it is what it is,
and the Hyper-V clocks do solve problems like migration that aren’t
addressed in ARM64 until v8.4 of the architecture with the addition of
the counter hardware scaling feature. Hyper-V doesn’t currently map the
ARM arch timer interrupts into guest VMs, so we need to use the existing
Hyper-V clocks and the common code that already exists.


Does the above answer your question?

Regards,
Boqun

> > 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.
> > 
> > ?
> > 
> 
> Option #1 would be the preferred solution, unless there is a good reason 
> against.
> 
> > Regards,
> > Boqun
> > 
> 
> -- 
> Regards,
> Vincenzo



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