Re: [UNVERIFIED SENDER] Re: [PATCH 0/9] arm64: Stolen time support

From: Alexander Graf
Date: Wed Aug 14 2019 - 10:52:50 EST



On 14.08.19 16:19, Marc Zyngier wrote:
On Wed, 14 Aug 2019 14:02:25 +0100,
Alexander Graf <graf@xxxxxxxxxx> wrote:



On 05.08.19 15:06, Steven Price wrote:
On 03/08/2019 19:05, Marc Zyngier wrote:
On Fri, 2 Aug 2019 15:50:08 +0100
Steven Price <steven.price@xxxxxxx> wrote:

Hi Steven,

This series add support for paravirtualized time for arm64 guests and
KVM hosts following the specification in Arm's document DEN 0057A:

https://developer.arm.com/docs/den0057/a

It implements support for stolen time, allowing the guest to
identify time when it is forcibly not executing.

It doesn't implement support for Live Physical Time (LPT) as there are
some concerns about the overheads and approach in the above
specification, and I expect an updated version of the specification to
be released soon with just the stolen time parts.

Thanks for posting this.

My current concern with this series is around the fact that we allocate
memory from the kernel on behalf of the guest. It is the first example
of such thing in the ARM port, and I can't really say I'm fond of it.

x86 seems to get away with it by having the memory allocated from
userspace, why I tend to like more. Yes, put_user is more
expensive than a straight store, but this isn't done too often either.

What is the rational for your current approach?

As I see it there are 3 approaches that can be taken here:

1. Hypervisor allocates memory and adds it to the virtual machine. This
means that everything to do with the 'device' is encapsulated behind the
KVM_CREATE_DEVICE / KVM_[GS]ET_DEVICE_ATTR ioctls. But since we want the
stolen time structure to be fast it cannot be a trapping region and has
to be backed by real memory - in this case allocated by the host kernel.

2. Host user space allocates memory. Similar to above, but this time
user space needs to manage the memory region as well as the usual
KVM_CREATE_DEVICE dance. I've no objection to this, but it means
kvmtool/QEMU needs to be much more aware of what is going on (e.g. how
to size the memory region).

You ideally want to get the host overhead for a VM to as little as you
can. I'm not terribly fond of the idea of reserving a full page just
because we're too afraid of having the guest donate memory.

Well, reduce the amount of memory you give to the guest by one page,
and allocate that page to the stolen time device. Problem solved!

Seriously, if you're worried about the allocation of a single page,
you should first look at how many holes we have in the vcpu structure,
for example (even better, with the 8.4 NV patches applied). Just
fixing that would give you that page back *per vcpu*.

I'm worried about additional memory slots, about fragmenting the cachable guest memory regions, about avoidable HV taxes.

I think we need to distinguish here between the KVM implementation and the hypervisor/guest interface. Just because in KVM we can save overhead today doesn't mean that the HV interface should be built around the assumption that "memory is free".


3. Guest kernel "donates" the memory to the hypervisor for the
structure. As far as I'm aware this is what x86 does. The problems I see
this approach are:

a) kexec becomes much more tricky - there needs to be a disabling
mechanism for the guest to stop the hypervisor scribbling on memory
before starting the new kernel.

I wouldn't call "quiesce a device" much more tricky. We have to do
that for other devices as well today.

And since there is no standard way of doing it, we keep inventing
weird and wonderful ways of doing so -- cue the terrible GICv3 LPI
situation, and all the various hacks to keep existing IOMMU mappings
around across firmware/kernel handovers as well as kexec.

Well, the good news here is that we don't have to keep it around ;).



b) If there is more than one entity that is interested in the
information (e.g. firmware and kernel) then this requires some form of
arbitration in the guest because the hypervisor doesn't want to have to
track an arbitrary number of regions to update.

Why would FW care?

Exactly. It doesn't care. Not caring means it doesn't know about the
page the guest has allocated for stolen time, and starts using it for
its own purposes. Hello, memory corruption. Same thing goes if you
reboot into a non stolen time aware kernel.

If you reboot, you go via the vcpu reset path which clears the map, no? Same goes for FW entry. If you enter firmware that does not set up the map, you never see it.



c) Performance can suffer if the host kernel doesn't have a suitably
aligned/sized area to use. As you say - put_user() is more expensive.

Just define the interface to always require natural alignment when
donating a memory location?

The structure is updated on every return to the VM.

If you really do suffer from put_user(), there are alternatives. You
could just map the page on the registration hcall and then leave it
pinned until the vcpu gets destroyed again.

put_user() should be cheap enough. It is one of the things we tend to
optimise anyway. And yes, worse case, we pin the page.


Of course x86 does prove the third approach can work, but I'm not sure
which is actually better. Avoid the kexec cancellation requirements was
the main driver of the current approach. Although many of the

I really don't understand the problem with kexec cancellation. Worst
case, let guest FW set it up for you and propagate only the address
down via ACPI/DT. That way you can mark the respective memory as
reserved too.

We already went down that road with the LPI hack. I'm not going there
again if we can avoid it. And it turn out that we can. Just allocate
the stolen time page as a separate memblock, give it to KVM for that
purpose.

Your suggestion of letting the guest firmware set something up only
works if whatever you're booting after that understands it. If it
doesn't, you're screwed.

Why? For UEFI, mark the region as reserved in the memory map. For DT, just mark it straight on reserved.

That said, I'm not advocating for doing it in the FW. I think this can be solved really easily with a simple guest driver to enable and a vcpu reset hook to disable the map.


But even with a Linux only mechanism, just take a look at
arch/x86/kernel/kvmclock.c. All they do to remove the map is to hook
into machine_crash_shutdown() and machine_shutdown().

I'm not going to take something that is Linux specific. It has to work
for all guests, at all times, whether they know about the hypervisor
service or not.

If they don't know about the HV service, they don't register the writer, so they don't see corruption.

If they know about the HV service and they don't support kexec, they don't have to worry because a vcpu reset should also clear the map.

If they do support kexec, they already have a mechanism to quiesce devices.

So I don't understand how this is Linux specific? The question was Linux specific, so I answered with precedence to show that disabling on kexec is not all that hard :).


Alex