Subject: RE: [RFC PATCH] powerpc/pseries/svm: capture instruction faulting on MMIO access, in sprg0 register

From: Michael Ellerman <[email-masked]>
Date: Fri, 24 Jul 2020 21:49:23 +1000

Ram Pai <[email-masked]> writes:
> On Wed, Jul 22, 2020 at 12:06:06PM +1000, Michael Ellerman wrote:
>> Ram Pai <[email-masked]> writes:
>> > An instruction accessing a mmio address, generates a HDSI fault.  This fault is
>> > appropriately handled by the Hypervisor.  However in the case of secureVMs, the
>> > fault is delivered to the ultravisor.
>> >
>> > Unfortunately the Ultravisor has no correct-way to fetch the faulting
>> > instruction. The PEF architecture does not allow Ultravisor to enable MMU
>> > translation. Walking the two level page table to read the instruction can race
>> > with other vcpus modifying the SVM's process scoped page table.
>> 
>> You're trying to read the guest's kernel text IIUC, that mapping should
>> be stable. Possibly permissions on it could change over time, but the
>> virtual -> real mapping should not.
>
> Actually the code does not capture the address of the instruction in the
> sprg0 register. It captures the instruction itself. So should the mapping
> matter?
>> 
>> > This problem can be correctly solved with some help from the kernel.
>> >
>> > Capture the faulting instruction in SPRG0 register, before executing the
>> > faulting instruction. This enables the ultravisor to easily procure the
>> > faulting instruction and emulate it.
>> 
>> This is not something I'm going to merge. Sorry.
>
> Ok. Will consider other approaches.

To elaborate ...

You've basically invented a custom ucall ABI. But a really strange one
which takes an instruction as its first parameter in SPRG0, and then
subsequent parameters in any GPR depending on what the instruction was.

The UV should either emulate the instruction, which means the guest
should not be expected to do anything other than execute the
instruction. Or it should be done with a proper ucall that the guest
explicitly makes with a well defined ABI.

cheers