Re: [PATCH] ppc/fadump: collect dump if the collected size is lesser than reserved

From: Sourabh Jain

Date: Tue Jul 28 2026 - 23:49:31 EST

On 22/07/26 14:41, Shivang Upadhyay wrote:

On Sun, 2026-07-19 at 12:39 +0530, Sourabh Jain wrote:

Could you please reword the commit title to make it a bit clearer?

On 14/07/26 23:00, Shivang Upadhyay wrote:

When a machine is subjected to CPUs add/remove, using dlpar
operations, the number of collected CPU_NOTES can change.
As per PAPR, collected dump size should not be more than
allocated size. Reflecting the same in source.

Can you add more details about the problem you are trying to solve
with this patch and how.

Adding the error message and scenario would be really helpful in
understanding the problem.

Hi Sourabh,

When a qemu ppc machine is booted with fadump=on and `-
smp=x,maxcpus=y`, on the panic kernel, /proc/vmcore is not generated
because dump_bytes and source_len does'nt match for CPU_STATE_DATA
region in fadump, then we just give up on parsing rest of the data.

Can you add Closes tag if it is reported upstream and if possible
fixes
tag too.

Signed-off-by: Shivang Upadhyay <shivangu@xxxxxxxxxxxxx>
---
arch/powerpc/platforms/pseries/rtas-fadump.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/platforms/pseries/rtas-fadump.c
b/arch/powerpc/platforms/pseries/rtas-fadump.c
index 3bb4ac2ab6cc..19a5adaf326b 100644
--- a/arch/powerpc/platforms/pseries/rtas-fadump.c
+++ b/arch/powerpc/platforms/pseries/rtas-fadump.c
@@ -469,7 +469,8 @@ static int __init rtas_fadump_process(struct
fw_dump *fadump_conf)
   pr_err("Dump taken by platform is
not valid (%d)\n", i);
   rc = -EINVAL;
   }
- if (fdm_active->rgn[i].bytes_dumped !=
fdm_active->rgn[i].source_len) {
+ if (be64_to_cpu(fdm_active-

rgn[i].bytes_dumped)

+     > be64_to_cpu(fdm_active-

rgn[i].source_len)) {

Can you please share your observations about `bytes_dump` for both
QEMU
and a
real system (LPAR) where the number of online CPUs is not equal to
the
maximum

I have the following observation.

I booted LPAR with 8 cpus. After crashing it I saw that fadump
CPU_STATE_DATA has notes for total 16 cpus, and only top 8 notes have
valid entries.when trying with 16, I see all NOTES have entried
filled.

Yes even I noticed the same on a LPAR with (Min=1 Desired=1 Max=2 with SMT=8 CPUs) maxcpus as 16 and online CPUs as 8.

[ 0.036128] rtas fadump: --------CPU State Data------------
[ 0.036129] rtas fadump: Magic Number: 5245475341564500
[ 0.036131] rtas fadump: NumCpuOffset: 1c
[ 0.036132] rtas fadump: NumCpus : 16
[ 0.036135] fadump: Allocated buffer for cpu notes of size 65536 at 0xc000000006a30000
[ 0.036138] rtas fadump: Reading register data for cpu 0...
[ 0.036173] rtas fadump: Reading register data for cpu 1...
[ 0.036178] rtas fadump: Reading register data for cpu 2...
[ 0.036209] rtas fadump: Reading register data for cpu 3...
[ 0.036240] rtas fadump: Reading register data for cpu 4...
[ 0.036272] rtas fadump: Reading register data for cpu 5...
[ 0.036304] rtas fadump: Reading register data for cpu 6...
[ 0.036335] rtas fadump: Reading register data for cpu 7...
[ 0.036390] rtas fadump: Updating elfcore header (c000000006a20000) with cpu notes

The NumCpus is populated to be 16 CPUs by the firmware (RTAS/PHYP) even though only 8 CPUs were online.

Kernel avoid processing reg entries of CPUs which were offline using below condition.
code snippet from rtas_fadump_build_cpu_notes()/rtas-faudmp.c

if (fdh && !cpumask_test_cpu(cpu, &fdh->cpu_mask)) {
RTAS_FADUMP_SKIP_TO_NEXT_CPU(reg_entry);
continue;
}

But when I removed the avoid condition kernel failed to process the reg entries and
below logs were printed.

[ 0.037877] rtas fadump: --------CPU State Data------------
[ 0.037879] rtas fadump: Magic Number: 5245475341564500
[ 0.037881] rtas fadump: NumCpuOffset: 1c
[ 0.037883] rtas fadump: NumCpus : 16
[ 0.037889] fadump: Allocated buffer for cpu notes of size 65536 at 0xc000000007a50000
[ 0.037891] rtas fadump: Reading register data for cpu 0...
[ 0.037937] rtas fadump: Reading register data for cpu 1...
[ 0.037974] rtas fadump: Reading register data for cpu 2...
[ 0.038009] rtas fadump: Reading register data for cpu 3...
[ 0.038044] rtas fadump: Reading register data for cpu 4...
[ 0.038079] rtas fadump: Reading register data for cpu 5...
[ 0.038082] rtas fadump: Reading register data for cpu 6...
[ 0.038117] rtas fadump: Reading register data for cpu 7...
[ 0.038155] rtas fadump: CPU 8 was offline
[ 0.038157] rtas fadump: Reading register data for cpu 8...
[ 0.038195] rtas fadump: CPU 10 was offline
[ 0.038197] rtas fadump: Reading register data for cpu 10...
[ 0.038235] rtas fadump: CPU 12 was offline
[ 0.038236] rtas fadump: Reading register data for cpu 12...
[ 0.038273] rtas fadump: CPU 14 was offline
[ 0.038275] rtas fadump: Reading register data for cpu 14...
[ 0.038311] rtas fadump: Unable to read CPU state data
[ 0.038319] fadump: Invalidating firmware-assisted dump registration
[ 0.038370] rtas fadump: Firmware busy during fadump invalidate, waiting 1ms (total 0ms)
[ 0.039403] fadump: reserved_memory_range[0] [0x00000040000000-0x000000800305c7], 0x400305c8 bytes
[ 0.039408] fadump: freeing reserved memory (0x80030000 - 0xa00000000)

For some reason, only CPUs with even-numbered IDs were present from offline CPUs list. This suggests that although
NumCpus was 16, only 8 CPUs had valid reg entries. I think we should discuss this case with the firmware team before
finalizing the solution.

On the other hand when I increased the CPUs count on the same system (Min=2 Desired=4 and max=8 with SMT 8) the
system was booted with 32 possible CPUs instead of 64 CPUs. And NumCpus was 32 in fadump kernel.

I previously proposed this fake entries fix in qemu [1]. But that is
not compliant with PAPR, which states that cpu notes should be
collected for current_cpus only.

Yes, that solution does not appear to be PAPR-compliant. However, based on our experiments, it is unclear why the firmware reports NumCpus as 16 when only 8 CPUs are actually online.

As per current states of things, reservation is always done for
maxcpus, (LPAR and QEMU both), NOTES collections is done for current
cpus on QEMU, and maxcpus on LPARs.

My proposal here is that we make the check for bytes_dumped and
source_len, less restrictive to let QEMU also generate /proc/vmcore,
when current_cpus != maxcpus.

By the way, the changes proposed in this patch also apply to other regions,
such as HPTE and REAL_MODE. That is not the intended behavior, right?

- Sourabh Jain

Regards
~Shivang.

[1]
https://lore.kernel.org/qemu-devel/20260520092857.1079193-1-shivangu@xxxxxxxxxxxxx/