Re: [PATCH v3] target: ppc: Use MSR_HVB bit to get the target endianness for memory dump

[Narayana Murty N <nnmlinux@linux.vnet.ibm.com>]

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On 5/22/23 23:50, Greg Kurz wrote:
> On Mon, 22 May 2023 12:02:42 -0400
> Narayana Murty N<nnmlinux@linux.ibm.com>  wrote:
>
>> Currently on PPC64 qemu always dumps the guest memory in
>> Big Endian (BE) format even though the guest running in Little Endian
>> (LE) mode. So crash tool fails to load the dump as illustrated below:
>>
>> Log :
>> $ virsh dump DOMAIN --memory-only dump.file
>>
>> Domain 'DOMAIN' dumped to dump.file
>>
>> $ crash vmlinux dump.file
>>
>> <snip>
>> crash 8.0.2-1.el9
>>
>> WARNING: endian mismatch:
>>            crash utility: little-endian
>>            dump.file: big-endian
>>
>> WARNING: machine type mismatch:
>>            crash utility: PPC64
>>            dump.file: (unknown)
>>
>> crash: dump.file: not a supported file format
>> <snip>
>>
>> This happens because cpu_get_dump_info() passes cpu->env->has_hv_mode
>> to function ppc_interrupts_little_endian(), the cpu->env->has_hv_mode
>> always set for powerNV even though the guest is not running in hv mode.
>> The hv mode should be taken from msr_mask MSR_HVB bit
>> (cpu->env.msr_mask & MSR_HVB). This patch fixes the issue by passing
>> MSR_HVB value to ppc_interrupts_little_endian() in order to determine
>> the guest endianness.
>>
>> The crash tool also expects guest kernel endianness should match the
>> endianness of the dump.
>>
>> The patch was tested on POWER9 box booted with Linux as host in
>> following cases:
>>
>> Host-Endianess Qemu-Target-Machine Qemu-Guest-Endianess  Qemu-Generated-Guest
>>                                                            Memory-Dump-Format
>> BE             powernv             LE KVM guest                 LE
>> BE             powernv             BE KVM guest                 BE
>> LE             powernv             LE KVM guest                 LE
>> LE             powernv             BE KVM guest                 BE
> I don't quite understand why KVM is mentioned with the powernv machine.

guest running mode was mentioned.

>
> Also have you tried to dump at various moments, e.g. during skiboot
> and when guest is booted, as in [1] which introduced the code this
> patch is changing ?
>
> [1]https://github.com/qemu/qemu/commit/5609400a422809c89ea788e4d0e13124a617582e.
>
>> LE             pseries KVM         LE KVM guest                 LE
>> LE             pseries TCG         LE guest                     LE
>>
> Fixes: 5609400a4228 ("target/ppc: Set the correct endianness for powernv memory dumps")

I agree, commit 5609400a4228 fixes endianness detection only for initial stage (skiboot) till endianness switch happens.
However, has_hv_mode is just a capability flag which is always set based on command-line param and doesnt really represent current hv state.
With this patch, it relies on the current state of the hv state based on the MSR_HVB of the msr_mask.

>
>> Signed-off-by: Narayana Murty N<nnmlinux@linux.ibm.com>
>> ---
>> Changes since V2:
>> commit message modified as per feedbak from Nicholas Piggin.
>> Changes since V1:
>> https://lore.kernel.org/qemu-devel/20230420145055.10196-1-nnmlinux@linux.ibm.com/
>> The approach to solve the issue was changed based on feedback from
>> Fabiano Rosas on patch V1.
>> ---
>>   target/ppc/arch_dump.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/target/ppc/arch_dump.c b/target/ppc/arch_dump.c
>> index f58e6359d5..a8315659d9 100644
>> --- a/target/ppc/arch_dump.c
>> +++ b/target/ppc/arch_dump.c
>> @@ -237,7 +237,7 @@ int cpu_get_dump_info(ArchDumpInfo *info,
>>       info->d_machine = PPC_ELF_MACHINE;
>>       info->d_class = ELFCLASS;
>>   
>> -    if (ppc_interrupts_little_endian(cpu, cpu->env.has_hv_mode)) {
>> +    if (ppc_interrupts_little_endian(cpu, !!(cpu->env.msr_mask & MSR_HVB))) {
>>           info->d_endian = ELFDATA2LSB;
>>       } else {
>>           info->d_endian = ELFDATA2MSB;

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