Re: [Qemu-ppc] [PATCH v3 3/4] target-ppc: ppc can be either endian

[Greg Kurz]

On Wed, 07 May 2014 13:54:36 +0200
Alexander Graf <address@hidden> wrote:
> On 05/07/2014 12:19 PM, Greg Kurz wrote:
> > On Wed, 7 May 2014 11:41:10 +0200
> > Alexander Graf <address@hidden> wrote:
> >
> >>
> >>> Am 07.05.2014 um 11:26 schrieb Peter Maydell <address@hidden>:
> >>>
> >>>> On 7 May 2014 10:09, Alexander Graf <address@hidden> wrote:
> >>>> I don't think we should overengineer hacks for legacy virtio.
> >>> Agreed. So what's our final conclusion: virtio endianness
> >>> is the endianness of the guest kernel at the point where
> >>> it triggers a reset of the virtio device, yes?
> >> I just realized we're talking about virtio in a non-virtio thread. This 
> >> patch set is about core dump support which is different from virtio 
> >> bi-endian support. While both may end up at the same logic, I don't like 
> >> the idea to mix them. This function is PPC internal.
> >>
> >> Alex
> >>
> > Correct and now I have this feeling about using LPCR_ILE versus MSR_LE...
> >
> > LPCR_ILE reflects the interrupt vector endianness. It is set during early 
> > boot
> > by the guest kernel according to the desired endianness. MSR_LE gives the
> > current endian mode for the cpu.
> >
> > The idea is that you need to rely on LPCR_ILE when you peek from the host
> > because you lack context and MSR_LE may be have been temporarily changed.
> > This is clearly the case for dump support.
> >
> > Now when it comes to virtio, we cache the endianness at device reset time: 
> > MSR_LE from
> > current_cpu should reflect the guest kernel endianness, no ?
> >
> > In this case we could end up like what's being currently discussed with ARM:
> >
> > http://www.spinics.net/lists/kvm-arm/msg09099.html
> > http://www.spinics.net/lists/kvm-arm/msg09091.html
> >
> > Alex,
> >
> > If we agree that current_cpu->MSR_LE does the job when the guest kernel 
> > resets
> > the device, then I guess we don't even need this patch...
> 
> Either one works for me as long as we put it into a spec. No solution 
> will be able to fulfill all cases.
> 
> The uglyness about the current_cpu bit is that devices are usually not 
> supposed to know about the cpu accesses come from usually. But then 
> again devices shouldn't know about the endianness of a cpu either so I 
> guess it's ok to breach layers here.
> 
> Rusty, do you have strong feelings either way?
> 
> 
> Alex
> 

Coming back to the initial subject. What about changing the last sentence
in the commit message to the following ?

"And when it comes to dumping a guest, the information is needed to write
 ELF headers using the kernel endianness."

This would make the patch a PPC64 dump only business and end the debate. :)

Tell me if you want me to repost.

Cheers.

-- 
Gregory Kurz                                     address@hidden
                                                 address@hidden
Software Engineer @ IBM/Meiosys                  http://www.ibm.com
Tel +33 (0)562 165 496

"Anarchy is about taking complete responsibility for yourself."
        Alan Moore.