Re: kexec failing with KVM on Power8 baremetal host

On 2/5/25 1:27 AM, Madhavan Srinivasan wrote:

On 2/5/25 12:35 AM, Stefan Berger wrote:

quoted

I bisected Linux between 6.13.0 and 6.12.0 due to failing kexec on a Power8 baremetal host on 6.13.0:

Actually, that's not correct what I wrote. The failure was inside a KVM 
VM running on a Power 8 host.

quoted

Can you please attach your config, also i cant recreate this host.

I attached it.

FYI, this here is the host:
processor       : 152
cpu             : POWER8E (raw), altivec supported
clock           : 2061.000000MHz
revision        : 2.1 (pvr 004b 0201)

timebase        : 512000000
platform        : PowerNV
model           : 8247-22L
machine         : PowerNV 8247-22L
firmware        : OPAL
MMU             : Hash

Maddy

quoted

8fec58f503b296af87ffca3898965e3054f2b616 is the first bad commit
commit 8fec58f503b296af87ffca3898965e3054f2b616
Author: Ritesh Harjani (IBM) [off-list ref]
Date:   Fri Oct 18 22:59:50 2024 +0530

     book3s64/hash: Add kfence functionality

     Now that linear map functionality of debug_pagealloc is made generic,
     enable kfence to use this generic infrastructure.

     1. Define kfence related linear map variables.
        - u8 *linear_map_kf_hash_slots;
        - unsigned long linear_map_kf_hash_count;
        - DEFINE_RAW_SPINLOCK(linear_map_kf_hash_lock);
     2. The linear map size allocated in RMA region is quite small
        (KFENCE_POOL_SIZE >> PAGE_SHIFT) which is 512 bytes by default.
     3. kfence pool memory is reserved using memblock_phys_alloc() which has
        can come from anywhere.
        (default 255 objects => ((1+255) * 2) << PAGE_SHIFT = 32MB)
     4. The hash slot information for kfence memory gets added in linear map
        in hash_linear_map_add_slot() (which also adds for debug_pagealloc).

     Reported-by: Pavithra Prakash [off-list ref]
     Signed-off-by: Ritesh Harjani (IBM) [off-list ref]
     Signed-off-by: Michael Ellerman [off-list ref]
     Link: https://patch.msgid.link/5c2b61941b344077a2b8654dab46efa0322af3af.1729271995.git.ritesh.list@gmail.com

  arch/powerpc/include/asm/kfence.h     |   5 ---
  arch/powerpc/mm/book3s64/hash_utils.c | 162 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++------
  2 files changed, 149 insertions(+), 18 deletions(-)


Reverting part of this patch by applying the following changes to 6.13.0 resolves the issue:

diff --git a/arch/powerpc/include/asm/kfence.h b/arch/powerpc/include/asm/kfence.h
index 1f7cab58ab2c..e7981719313c 100644
--- a/arch/powerpc/include/asm/kfence.h
+++ b/arch/powerpc/include/asm/kfence.h

@@ -10,6 +10,7 @@

  #include <linux/mm.h>
  #include <asm/pgtable.h>
+#include <asm/mmu.h>

  #ifdef CONFIG_PPC64_ELF_ABI_V1
  #define ARCH_FUNC_PREFIX "."

@@ -25,6 +26,10 @@ static inline void disable_kfence(void)

  static inline bool arch_kfence_init_pool(void)
  {
+#ifdef CONFIG_PPC64
+       if (!radix_enabled())
+               return false;
+#endif
         return !kfence_disabled;
  }