Re: [PATCH] ptp: Add vDSO-style vmclock support

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 25, 2024 at 04:18:43PM +0100, David Woodhouse wrote:
> On Thu, 2024-07-25 at 10:11 -0400, Michael S. Tsirkin wrote:
> > On Thu, Jul 25, 2024 at 02:50:50PM +0100, David Woodhouse wrote:
> > > Even if the virtio-rtc specification were official today, and I was
> > > able to expose it via PCI, I probably wouldn't do it that way. There's
> > > just far more in virtio-rtc than we need; the simple shared memory
> > > region is perfectly sufficient for most needs, and especially ours.
> > 
> > I can't stop amazon from shipping whatever in its hypervisor,
> > I'd just like to understand this better, if there is a use-case
> > not addressed here then we can change virtio to address it.
> > 
> > The rtc driver patch posted is 900 lines, yours is 700 lines, does not
> > look like a big difference.  As for using a memory region, this is
> > valid, but maybe rtc should be changed to do exactly that?
> 
> I'm certainly aiming for virtio-rtc to include that as an *option*,
> because I think I don't think it makes sense for an RTC specification
> aimed at virtual machines *not* to deal with the live migration
> problem.
> 
> AFAICT the only ways to deal with the LM problem are either to make a
> hypercall/virtio transaction for *every* clock read which needs to be
> accurate, or expose a memory region for the guest to do it "vDSO-
> style".

virtio can support the second option, we already have
VIRTIO_PCI_CAP_SHARED_MEMORY_CFG, I'd just use it.


> And similarly, unless we want guest userspace to have to make a
> *system* call every time, that memory region needs to be mappable all
> the way to userspace.

This part is classic for pci, mapping pci bar has been well
studied.


> The use case isn't necessarily for all users of gettimeofday(), of
> course; this is for those applications which *need* precision time.
> Like distributed databases which rely on timestamps for coherency, and
> users who get fined millions of dollars when LM messes up their clocks
> and they put wrong timestamps on financial transactions.

I would however worry that with all this pass through,
applications have to be coded to each hypervisor or even
version of the hypervisor.

I don't really know the use-case well enough - is sending
an interrupt to linux and having linux create a device
independent structure not workable?


> > E.g. we can easily add a capability describing such a region.
> > or put it in device config space.
> 
> I think it has to be memory, not config space. But yes.

virtio config space, which is just a region in a BAR.
But yes, maybe VIRTIO_PCI_CAP_SHARED_MEMORY_CFG is cleaner.

> The intent is that my driver would be usable with the shared memory
> region from a virtio-rtc device too. It'd need a tiny amount of
> refactoring of the discovery code in vmclock_probe(), which I haven't
> done yet as it would be premature optimisation. 
> 
> > I mean yes, we can build a new transport for each specific need but in
> > the end we'll get a ton of interfaces with unclear compatibility
> > requirements.  If effort is instead spent improving common interfaces,
> > we get consistency and everyone benefits. That's why I'm trying to
> > understand the need here.
> 
> It's simplicity. Because this isn't even a "transport". It's just a
> simple breadcrumb given to the guest to tell it where the information
> is.
> In the fullness of time assuming this becomes part of virtio-rtc too,
> the fact that it can *also* be discovered by ACPI is just a tiny
> detail. And it allows hypervisors to implement it a *whole* lot more
> simply.
> 
> The addition of an ACPI method to enable the timekeeping does make it a
> tiny bit more than a 'breadcrump', I concede — but that's still
> basically trivial to implement. A whole lot simpler than a full virtio
> device.

virtio has been developed with the painful experience that we keep
making mistakes, or coming up with new needed features,
and that maintaining forward and backward compatibility
becomes a whole lot harder than it seems in the beginning.

-- 
MST