[PATCH 1/3] powerpc/mce: remove nmi_enter/exit from real mode handler

[Michal Suchánek]

Hello,

On Wed, Sep 16, 2020 at 10:52:26PM +0530, Ganesh Goudar wrote:
> Use of nmi_enter/exit in real mode handler causes the kernel to panic
> and reboot on injecting slb mutihit on pseries machine running in hash
> mmu mode, As these calls try to accesses memory outside RMO region in
> real mode handler where translation is disabled.
> 
> Add check to not to use these calls on pseries machine running in hash
> mmu mode.
> 
> Fixes: 116ac378bb3f ("powerpc/64s: machine check interrupt update NMI accounting")
> Signed-off-by: Ganesh Goudar <ganeshgr at linux.ibm.com>
> ---
>  arch/powerpc/kernel/mce.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kernel/mce.c b/arch/powerpc/kernel/mce.c
> index ada59f6c4298..1d42fe0f5f9c 100644
> --- a/arch/powerpc/kernel/mce.c
> +++ b/arch/powerpc/kernel/mce.c
> @@ -591,10 +591,15 @@ EXPORT_SYMBOL_GPL(machine_check_print_event_info);
>  long notrace machine_check_early(struct pt_regs *regs)
>  {
>  	long handled = 0;
> -	bool nested = in_nmi();
> +	bool nested;
> +	bool is_pseries_hpt_guest;
>  	u8 ftrace_enabled = this_cpu_get_ftrace_enabled();
>  
>  	this_cpu_set_ftrace_enabled(0);
> +	is_pseries_hpt_guest = machine_is(pseries) &&
> +			       mmu_has_feature(MMU_FTR_HPTE_TABLE);
> +	/* Do not use nmi_enter/exit for pseries hpte guest */
> +	nested = is_pseries_hpt_guest ? true : in_nmi();
As pointed out already in another comment nesting is supported natively
since 69ea03b56ed2c7189ccd0b5910ad39f3cad1df21. You can simply do
nmi_enter and nmi_exit unconditionally - or only based on
is_pseries_hpt_guest.

The other question is what is the value of calling nmi_enter here at
all. It crashes in one case, we simply skip it for that case, and we are
good. Maybe we could skip it altogether?

Thanks

Michal