Re: [PATCH] target: ppc: Correctly initialize HILE in HID-0 for book3s processors

[Fabiano Rosas]

Vaibhav Jain <vaibhav@linux.ibm.com> writes:

> Hi Fabiano,
>
> Thanks for looking into this patch and apologies for the delayed reponse.
> Fabiano Rosas <farosas@suse.de> writes:
>
>> Narayana Murty N <nnmlinux@linux.ibm.com> writes:
>>
>>> On PPC64 the HILE(Hypervisor Interrupt Little Endian) bit in HID-0
>>> register needs to be initialized as per isa 3.0b[1] section
>>> 2.10. This bit gets copied to the MSR_LE when handling interrupts that
>>> are handled in HV mode to establish the Endianess mode of the interrupt
>>> handler.
>>>
>>> Qemu's ppc_interrupts_little_endian() depends on HILE to determine Host
>>> endianness which is then used to determine the endianess of the guest dump.
>>>
>>
>> Not quite. We use the interrupt endianness as a proxy to guest
>> endianness to avoid reading MSR_LE at an inopportune moment when the
>> guest is switching endianness.
> Agreed
>
>> This is not dependent on host
>> endianness. The HILE check is used when taking a memory dump of a
>> HV-capable machine such as the emulated powernv.
>
> I think one concern which the patch tries to address is the guest memorydump 
> file
> generated of a BigEndian(BE) guest on a LittleEndian(LE) host is not readable 
> on
> the same LE host since 'crash' doesnt support cross endianess
> dumps. Also even for a LE guest on LE host the memory dumps are marked as BE
> making it not possible to analyze any guest memory dumps on the host.
>

>From QEMU's perspective there's no "host" in this equation. We'll
generate a BE dump for a BE guest and a LE dump for a LE guest. Anything
different is a bug in QEMU (as the one this patch addresses).

> However setting the HILE based on host endianess of qemu might not be
> the right way to fix this problem. Based on an off mailing list discussion
> with Narayana, he is working on another patch which doesnt set HILE
> based on host endianess. However the problem seems to be stemming from
> fact that qemu on KVM is using the HILE to set up the endianess of
> memory-dump elf and since its not setup correctly the memory dumps are
> in wrong endianess.
>
>> I think the actual issue might be that we're calling
>> ppc_interrupts_little_endian with hv=true for the dump.
>>
> Yes, that is currently the case with cpu_get_dump_info(). Excerpt from
> that function below that sets the endianess of the dump:
>
>     if (ppc_interrupts_little_endian(cpu, cpu->env.has_hv_mode)) {

This should probably be looking at cpu->vhyp or MSR_HVB since
has_hv_mode will not change after we init the cpu.

>         info->d_endian = ELFDATA2LSB;
>     } else {
>         info->d_endian = ELFDATA2MSB;
>     }
>
> for pseries kvm guest cpu->env.has_hv_mode is already set hence
> ppc_interrupts_little_endian() assumes its running in 'hv' mode. The new
> patch from Narayana will be addressing this.
>
>>> Currently the HILE bit is never set in the HID0 register even if the
>>> qemu is running in Little-Endian mode. This causes the guest dumps to be
>>> always taken in Big-Endian byte ordering. A guest memory dump of a
>>> Little-Endian guest running on Little-Endian qemu guest fails with the
>>> crash tool as illustrated below:
>>>
>>
>> Could you describe in more detail what is your setup? Specifically
>> whether both guests are running TCG or KVM (info kvm) and the state of
>> the nested-hv capability in QEMU command line.
> Currently the issue is seen with any pseries KVM guest running on a PowerNV 
> host.