Re: [Qemu-devel] [PATCH 1/7] virtio: allow byte swapping for vring and config access

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Mon, 2013-08-12 at 09:58 +0930, Rusty Russell wrote:
> Benjamin Herrenschmidt <benh@kernel.crashing.org> writes:
> > This whole exercise should have nothing to do with the current endian
> > mode of the CPU. If for example you are running lx86 (the x86 emulator
> > IBM provides) which exploits MSR:LE on POWER7 to run x86 binaries in
> > userspace, you don't want virtio to suddenly change endian !
> >
> > The information we care about is the endianness of the operating system.
> 
> Which is why my original patches nabbed the endianness when the target
> updated the virtio device status.
> 
> You're making an assumption about the nature of the guest, that they
> don't pass the virtio device directly through to userspace.

Two points here:

 - Userspace is VERY likely to have the same endianness as the operating
system.

 - The case where we might support "foreign endian" userspace *and* pass
virtio directly to it *and* give a shit about virtio v1.0 doesn't exist
anywhere but your imagination right now :-)

> I don't care, though.  The point is to make something which works, until
> the Real Fix (LE virtio).

Exactly.

> > The most logical way to infer it is a different bit, which used to be
> > MSR:ILE and is now in LPCR for guests and controlled via a hypercall on
> > pseries, which indicates what is the endianness of interrupt vectors.
> >
> > IE. It indicates how the cpu should set MSR:LE when taking an interrupt,
> > regardless of what the current MSR:LE value is at any given point in
> > time.
> >
> > So what should be done in fact is whenever *that* bit is changed
> > (currently via hcall, maybe via MSR:ILE if we emulate that on older
> > models or LPCR when we emulate that), then the qemu cpu model can "call
> > out" to change the "OS endianness" which we can propagate to virtio.
> >
> > Anything trying to do stuff based on the "current" endianness in the MSR
> > sounds like a cesspit to me.
> 
> OK.  What should Anton's gdb stub do then?

Something else. It's a different problem and needs a different solution.

For one, I think, we should first fix the root problem with gdb (tagging
endianness in the protocol etc...) and once that's done, look at what
band-aid can be applied for old stuff if we care at all (it's not like
LE ppc64 is going to not require a new gdb anyway).

Cheers,
Ben.