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

On 9/17/20 5:50 PM, Michal Suchánek wrote:

Hello,

On Wed, Sep 16, 2020 at 10:52:26PM +0530, Ganesh Goudar wrote:

quoted

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 <redacted>
---
  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.

ok

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?

Not sure why nmi_enter/exit is needed here, Again, Nick may have a reason.

Thanks

Michal