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

[Cédric Le Goater]

On 5/22/23 18:02, Narayana Murty N wrote:
> Currently on PPC64 qemu always dumps the guest memory in
> Big Endian (BE) format even though the guest running in Little Endian



The patch is surely correct. I have problems understanding the config
you are testing. PPC Book3s has multiple hypervisor implementations :

1. pHyp (AKA PowerVM)
2. OPAL/PowerNV (AKA Power KVM-HV)
3. OPAL/PowerNV/pSeries (AKA Power KVMHV-on-pSeries)
4. pHyp/pSeries (very recent implementation, I don't know how it is
   referred to in the kernel)

I am leaving the KVM-PR implementation out of the discussions for
simplicity.

QEMU also supports emulation of 2. and 3. in two different machines
PowerNV and pseries, although running pseries guests under a PowerNV
machine is slow, so is running pseries guests under pseries.

Could you please describe your environment ?

Thanks,

C.



> (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
> LE             pseries KVM         LE KVM guest                 LE
> LE             pseries TCG         LE guest                     LE
>
> 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;