Re: [PATCH 00/13] arm64: Virtualization Host Extension support

[Antonios Motakis]

On 26-Aug-15 11:59, Marc Zyngier wrote:
> On 26/08/15 10:21, Jan Kiszka wrote:
>> On 2015-08-26 11:12, Antonios Motakis wrote:
>>> Hello Marc,
>>>
>>> On 08-Jul-15 18:19, Marc Zyngier wrote:
>>>> ARMv8.1 comes with the "Virtualization Host Extension" (VHE for 
>>>> short), which enables simpler support of Type-2 hypervisors.
>>>>
>>>> This extension allows the kernel to directly run at EL2, and 
>>>> significantly reduces the number of system registers shared
>>>> between host and guest, reducing the overhead of virtualization.
>>>>
>>>> In order to have the same kernel binary running on all versions
>>>> of the architecture, this series makes heavy use of runtime code
>>>> patching.
>>>>
>>>> The first ten patches massage the KVM code to deal with VHE and
>>>> enable Linux to run at EL2.
>>>
>>> I am currently working on getting the Jailhouse hypervisor to work
>>> on AArch64.
>>>
>>> I've been looking at your patches, trying to figure out the
>>> implications for Jailhouse. It seems there are a few :)
>>>
>>> Jailhouse likes to be loaded by Linux into memory, and then to
>>> inject itself at a higher level than Linux (demoting Linux into
>>> being the "root cell"). This works on x86 and ARM (AArch32 and
>>> eventually AArch64 without VHE). What this means in ARM, is that
>>> Jailhouse hooks into the HVC stub exposed by Linux, and happily
>>> installs itself in EL2.
>>>
>>> With Linux running in EL2 though, that won't be as straightforward.
>>> It looks like we can't just demote Linux to EL1 without breaking
>>> something. Obviously it's OK for us that KVM won't work, but it
>>> looks like at least the timer code will break horribly if we try to
>>> do something like that.
>>>
>>> Any comments on this? One work around would be to just remap the
>>> incoming interrupt from the timer, so Linux never really realizes
>>> it's not running in EL2 anymore. Then we would also have to deal
>>> with the intricacies of removing and re-adding vCPUs to the Linux
>>> root cell, so we would have to maintain the illusion of running in
>>> EL2 for each one of them.
> 
> Unfortunately, there is more to downgrading to EL1 than just interrupts.
> You need to migrate the whole VM context from EL2 to EL1 in an atomic
> fashion, clear the HCR_EL2.E2H and HCR_EL2.TGE bits while running at EL2
> (which is a bit like pulling the rug from under your own feet so you
> need to transition via a low mapping or an idmap), reinstall the EL2
> stub and do an exception return into EL1.

When enabling Jailhouse, we already do most of that. We already use identity mapping, since we need to switch on the MMU for EL2, switch the exception level, etc. Jailhouse entry looks a lot like initializing a new kernel; we just save the state of what was running before it and restore it as the "root cell".

So I think we could handle the cpu context switch, with changes only in the Jailhouse entry code. But then of course, Linux would be expecting to be in EL2, while it is running in EL1, so we would have to emulate the differences in behavior. But...

> 
> And that's only for the CPU. Downgrading to EL1 has other fun
> consequences at the system level (SMMUs listening to TLB traffic would
> need to be reconfigured on the flight - it's a joke, don't even think of
> it).

...but then there's that.

Hm... even if the kernel is running in EL2, it will still be configuring stage 1 on the SMMU, no? I wonder if this could still be handled somehow... The root cell would be restored with identity mapping, too... Just thinking out loud :)

> 
>>
>> Without knowing any of the details, I would say there are two
>> strategies regarding this:
>>
>> - Disable KVM support in the Linux kernel - then we shouldn't boot
>> into EL2 in the first place, should we?
> 
> Disabling KVM support won't drop the kernel to EL1. At least that's not
> what the current code does (we stay at EL2 if we detect VHE), and that's
> way too early to be able to parse a command line option.
> 
>> - Emulate what Linux is missing after take-over by Jailhouse (we do 
>> this on x86 with VT-d interrupt remapping which cannot be disabled 
>> anymore for Linux once it started with it, and we cannot boot
>> without it when we want to use the x2APIC).
> 
> I can't really see what you want to emulate (I'm afraid I'm lacking the
> x86-specific background).
> 
> As far as I can see, the only practical solution to this is to have a
> VHE config option, and Jailhouse that can be set to conflict it (depends
> on !VHE).

Having a toggle to turn VHE off at build time would definitely be the easy way out. Then we can just tell the user that we only support kernels built without it (the Jailhouse driver is out of tree atm).

I don't have access to a VHE model though. Are you considering to add a config option for VHE in the next version of your patches?

In any case thanks for your thoughts,

-- 
Antonios Motakis
Virtualization Engineer
Huawei Technologies Duesseldorf GmbH
European Research Center
Riesstrasse 25, 80992 München

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