Re: [PATCH] powerpc/64: Don't try to use radix MMU under a hypervisor

On Tue, Dec 20, 2016 at 08:19:02PM +0530, Aneesh Kumar K.V wrote:

Paul Mackerras [off-list ref] writes:

quoted

Currently, if the kernel is running on a POWER9 processor under a
hypervisor, it will try to use the radix MMU even though it doesn't
have the necessary code to use radix under a hypervisor (it doesn't
negotiate use of radix, and it doesn't do the H_REGISTER_PROC_TBL
hcall).  The result is that the guest kernel will crash when it tries
to turn on the MMU, because it will still actually be using the HPT
MMU, but it won't have set up any SLB or HPT entries.  It does this
because the only thing that the kernel looks at in deciding to use
radix, on any platform, is the ibm,pa-features property on the cpu
device nodes.

This fixes it by looking for the /chosen/ibm,architecture-vec-5
property, and if it exists, clearing the radix MMU feature bit.
We do this before we decide whether to initialize for radix or HPT.
This property is created by the hypervisor as a result of the guest
calling the ibm,client-architecture-support method to indicate
its capabilities, so it only exists on systems with a hypervisor.
The reason for using this property is that in future, when we
have support for using radix under a hypervisor, we will need
to check this property to see whether the hypervisor agreed to
us using radix.

Hypervisor that doesn't support radix should clear the ibm,pa-features
radix feature bit right ? We look at that before setting
MMU_FTR_TYPE_RADIX. So how did we end up enabling radix in the above
case ?

Two points in response:

1. The ibm,pa-features property specifies the capabilities of the
processor, not the platform.  The radix bit would be set on POWER9
even if the hypervisor doesn't support radix.

2. The hypervisor might well support radix, but the kernel definitely
cannot operate with radix under a hypervisor, because it doesn't put
the necessary things in the ibm,client-architecture support call, and
it doesn't call the H_REGISTER_PROC_TBL hcall.  That means that the
CPU has no way to find our 1st level (process scoped) radix trees even
if the partition was in radix mode.  Most likely the hypervisor will
put the partition into HPT mode because the kernel didn't ask for
radix in the CAS call, but even if the hypervisor put the partition
into radix mode, it still wouldn't work.

Paul.