[RFC] powerpc/emulate_step: Fix kernel crash when VSX is not present

[Naveen N. Rao]

[Adding Mikey]

Ravi Bangoria wrote:
> emulate_step() is not checking runtime VSX feature flag before
> emulating an instruction. This can cause kernel oops when kernel
> is compiled with CONFIG_VSX=y but running on machine where VSX is
> not supported or disabled. Ex, while running emulate_step tests on
> P6 machine:
> 
>   ...
>   emulate_step_test: lvx            : PASS
>   emulate_step_test: stvx           : PASS
>   Oops: Exception in kernel mode, sig: 4 [#1]
>   NIP [c000000000095c24] .load_vsrn+0x28/0x54
>   LR [c000000000094bdc] .emulate_loadstore+0x167c/0x17b0
>   Call Trace:
>   [c0000000494c3770] [40fe240c7ae147ae] 0x40fe240c7ae147ae (unreliable)
>   [c0000000494c37f0] [c000000000094bdc] .emulate_loadstore+0x167c/0x17b0
>   [c0000000494c3900] [c000000000094f6c] .emulate_step+0x25c/0x5bc
>   [c0000000494c39b0] [c000000000ec90dc] .test_lxvd2x_stxvd2x+0x64/0x154
>   [c0000000494c3b90] [c000000000ec9204] .test_emulate_step+0x38/0x4c
>   [c0000000494c3c00] [c00000000000de2c] .do_one_initcall+0x5c/0x2c0
>   [c0000000494c3cd0] [c000000000eb4bf8] .kernel_init_freeable+0x314/0x4cc
>   [c0000000494c3db0] [c00000000000e1e4] .kernel_init+0x24/0x160
>   [c0000000494c3e30] [c00000000000bc24] .ret_from_kernel_thread+0x58/0xb4
> 
> With fix:
>   ...
>   emulate_step_test: stvx           : PASS
>   emulate_step_test: lxvd2x         : FAIL
>   emulate_step_test: stxvd2x        : FAIL

Those tests should also be updated to skip testing these instructions if 
!CPU_FTR_VSX.

> 
> Fixes: https://github.com/linuxppc/linux/issues/148
> 
> Reported-by: Michael Ellerman <mpe at ellerman.id.au>
> Signed-off-by: Ravi Bangoria <ravi.bangoria at linux.ibm.com>
> ---
> Note: VSX was first introduced on P6. But there are many VSX instructions
>       which were introduced in later versions. So ideally, analyse_instr()
>       should check for ISA version along with opcode.

Yes, this patch should be useful for when VSX is not present. For 
specific instructions, we should add a cpu feature check.

- Naveen