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Re: [Qemu-devel] ARM QEMU/KVM and TrustZone

From: Антон Кочков
Subject: Re: [Qemu-devel] ARM QEMU/KVM and TrustZone
Date: Sat, 16 Jun 2012 23:49:58 +0400

Hm, one memory space, but what about write access restrictions, e.g.
for Non-Secure or Secure worlds for some memory addresses/blocks?

Best regards,
Anton Kochkov.

On Sat, Jun 16, 2012 at 9:37 PM, Christoffer Dall
<address@hidden> wrote:
>> On 22 May 2012 13:22, Peter Maydell <address@hidden> wrote:
>>> Historically for QEMU we haven't implemented TrustZone support even
>>> though we claim to emulate CPUs that provide it. Instead we provide a
>>> CPU which mostly looks like a variant of the real thing without the
>>> TrustZone feature. We then bolt on a few extra cp15 registers (eg the
>>> SCR) as a pragmatic move to get Linux guests to run. Now we're also
>>> dealing with KVM on ARM I'd like to define things a bit more solidly
>>> so KVM and TCG agree on what the CPU model they present is.
>>> There are several possible environments we could provide
>>> to a guest:
>>>  (1) a CPU with full TrustZone support
>>>  (2) a CPU without TrustZone at all
>>>  (3) a TZ CPU running in NonSecure PL0/PL1
>>>  (4) a TZ CPU running in Secure PL0/PL1
>>> In some ways (1) is the "purist" solution -- emulate exactly what the
>>> hardware does. However:
>>>  * on TCG it would require a lot of work, including new functionality
>>>   in core QEMU (to support having different CPU cores being able to
>>>   see different views of memory, and having the S/NS attribute
>>>   attached to memory transactions)
>>>  * it isn't possible in KVM, because the ARM Virtualization Extensions
>>>   don't allow you to fake the CPSR a guest sees, and so you can't
>>>   make the guest believe it is in Monitor mode
>>> Option (2) is architecturally sanctioned (ie TrustZone is an optional
>>> feature, not mandatory), but it doesn't correspond to real CPUs, in
>>> that the hardware Cortex-A8/A9/A15 always have TrustZone. So we're
>>> modelling something that doesn't really exist.
>>> Options (3) and (4) correspond to the environment an OS guest
>>> typically actually uses on hardware. For ARM's devboards (versatile
>>> express etc) Linux runs in the Secure world but it doesn't actually
>>> use any of the TrustZone functionality, it's just a "give me full
>>> access to everything" setup. For just about every other ARM system,
>>> the boot rom or equivalent keeps Secure world to itself, and the OS
>>> kernel runs in the NonSecure world. (This typically means that the
>>> boot rom provides a set of board-specific entry points via the Secure
>>> Monitor Call (SMC) instruction for doing operations like "invalidate
>>> whole L2 cache" which require secure privileges.)
> Is there anything preventing people from writing a small bootloader
> that switches into non-secure mode and runs kernels there as a general
> approach (apart from laziness)?
>>> Proposal:
>>> My suggestion is that we present the guest with a view that looks like
>>> a sort of superset of (2) (3) and (4), ie sufficient that a guest
>>> expecting any of those environments can run. In particular:
>>>  * no cp15 registers have secure/nonsecure banking
>>>  * there is only one memory space visible
>>>  * secure-access-only permissions are not enforced
>>>  * the handful of only-in-trustzone registers are implemented
>>>   (eg VBAR, MVBAR)
>>>  * we implement a "fake monitor mode"
>>> The aim of the "fake monitor mode" is to allow us to provide fake
>>> qemu-specific bootroms which implement whatever the board's SMC
>>> interface is, without having to write specific KVM kernel code for
>>> each board. So we don't have to run arbitrary secure-world guest code.
>>> The rules are:
>>>  * on an SMC instruction we enter the guest at the SMC vector
>>>   as defined by the MVBAR (monitor vector base address register)
>>>  * we actually run with the same access permissions as above
>>>   (and under KVM if you look at CPSR.M it will tell you you're
>>>   in Supervisor mode)
>>>  * return from the SMC is via a standard exception return insn
>>>  * we don't implement the separate memory space for the secure
>>>   world. (This implies that you need to find space in the
>>>   non-secure world's physical memory map for the bootrom shim;
>>>   not a big deal I think since we already have a requirement
>>>   for some space to put QEMU's arm_boot trivial bootloader.)
> you could have a separate set of stage-2 translation tables for this
> and keep things separate for real (or would we rely on fake-SMC code
> to directly be able to read fake-non-secure data, which is still
> possible through a different memory map I guess).
>>> The code written for this fake monitor mode environment is likely to
>>> be able to work OK if we ever implement full TrustZone support in TCG
>>> QEMU.
> what kind of operations would be required from SMC calls in a KVM
> guest setting? I can see this in an embedded market, but are they not
> likely to even capture Hyp mode already and set things up as required?
> What I mean is, if KVM is currently targeting Calxeda-type setups will
> we ever run kernels that require SMC operations as guests?
> It feels a bit premature to implement all this.
>>> Work required:
>>> * Documentation: the general principles as listed above
>>> * TCG: make sure we have implementations of all the TZ registers
>>> * TCG: implement the SMC and fake-monitor-mode
>>>  (I already have patches from Nokia in the qemu-linaro
>>>  stack which can be cleaned up and used here)
>>> * KVM: implement emulation of MVBAR
>>> * KVM: set the config bit so SMC is trapped to the hypervisor
>>>  and causes guest restart at the right entrypoint
>>> * KVM: if there turns out to be anything that fake-monitor-mode
>>>  needs to do that requires Hyp privilege we'd need a hypercall
>>>  ABI, but I can't currently think of anything
>>> I think that's basically a fairly small set of work to formalise
>>> the approach we're already taking in practice, and make it a little
>>> more flexible.
>>> Opinions?
> It feels like quite a bit of complexity at this point. But if we need
> it, we need it. I'm just not convinced of that yet.
> -Christoffer

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