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Re: [Xen-devel] [ARM] Native application design and discussion (I hope)


On 10/05/17 20:04, Julien Grall wrote:
On 05/10/2017 06:37 PM, Volodymyr Babchuk wrote:
Hi Julien,

Hi Volodymyr,

Returning back to Native apps, I think we can make ctx switch even
faster by dropping p2m code. Imagine that we already created stage 1
MMU for native application. Then to switch to app it we need only:

1. Enable TGE bit in HCR
2. Disable VM bit in HCR
3. Save/Program EL1_TTBR and friends
3.5 (optionally) save/restore FPU state
4. Save/Restore general purpose registers + SP + CSR + PC to jump to
an app in EL0 state.

This can be done in "real" vcpu or in idle vcpu context. No
differences there.

Exception handling in hypervisor would became tricky because of vcpu
absence for native app. Current implementation of entry.S always says
general purpose registers to a vcpu structure. Basically, we should
teach entry.S and traps.c about native apps.
Am I missing something?

HCR_EL2.VM is allowed to be cached in the TLBs so for correctness you
have to flush the TLBs everytime you change this bit (see D4.8.3 in ARM
DDI 0487A.k_iss10775).

Furthermore, as I mentioned earlier (see [1]) there are dependencies on
the VMID even when stage-2 is disabled (see D4-1823 in ARM DDI
0487A.k_iss10775) so you have to program correctly VTTBR_EL2.VMID. This
also means that if you use a different EL0 app, you have to ther use a
different VMID or flush the TLBs.

Bottom line, if you don't use stage-2 page table you have to flush the
TLBs. Likely this will have an higher impact on the platform than using
stage-2 page table.

Virtual memory is quite tricky, someone needs to look at the ARM ARM and
check all the behaviors when disabling either stage-1 or stage-2. There
are memory attribute implications that may make tricky to move an EL0
app between pCPU.

Looking again at the documentation and chatting with other ARM folks. I was wrong on some part, sorry for the confusion.

It turns out that if you don't need to flush the TLBs when disabling the HCR_EL2.VM (this is what Linux does for KVM). So disabling stage-2 for EL0 app would be ok.

But you still need to allocate a VMID per EL0 app as TLBs will still depend on it even with stage-2 disabled.

Even if we keep stage-2 enabled, we would have to create dummy page tables of stage-1 because the memory attribute would impact performance and at least not allow the EL0 app to move (see D4.2.8 in ARM DDI 0487A.k_iss10775). In this case, 1:1 page tables with a block map (e.g 1GB) would be sufficient and rely on stage-2 page tables.

Lastly, can you remind me with platform you are using for testing?

I hope this helps.


Julien Grall

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