Re: [Qemu-devel] [Qemu-ppc] KVM and variable-endianness guest CPUs

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

On Mon, 2014-01-27 at 16:44 -0800, Christoffer Dall wrote:

> I'm loosing track of this discussion, Ben, can you explain a bit?  You
> wrote:
> 
>   Having a byte array coming in that represents what the CPU does in its
>   current byte order means you do *NOT* need to query the endianness of
>   the guest CPU from userspace.
> 
> What does "a byte array that represents what the CPU does in its current
> byte order" mean in this context.  Do you mean the VCPU or the physical
> CPU when you say CPU.

It doesn't matter once it's a byte array in address order. Again this is
the *right* abstraction for the kernel ABI, because you do not care
about the endianness of either side, guest or host.

It makes no sense to treat a modern CPU data bus as having an MSB and an
LSB (even if they have it sometimes on the block diagram). Only when
*interpreting a value* on that bus, such as an *address* does the
endianness become of use.

Treat the bus instead as an ordered sequence of bytes in ascending
address order and most of the complexity goes away.

>From there, for a given device, it all depends which bytes *that device*
choses to consider as being the MSB vs. LSB. It's not even a bus thing,
though of course some busses suggest an endianness, and some like PCI
mandates it for configuration space. 

But it remains a device-side choice.

> I read your text as saying "just do a store of the register into the
> data pointer and don't worry about endianness", but somebody, somewhere,
> has to check the VCPU endianness setting.
> 
> I'm probably wrong, and you are probably the right person to clear this
> up, but can you formulate exactly what you think the KVM ABI is and how
> you would put it in Documentation/virtual/kvm/api.txt?
> 
> My point of view is that it is KVM that needs to do this, and it should
> "emulate the CPU" by performing a byteswap in the case where the CPU
> E-bit is set on ARM, but this is an ARM-centric way of looking at
> things.

The ABI going to qemu should be (and inside qemu from TCG to the
emulation) that the CPU did an access of N bytes wide at address A
whose value is the byte array data[] in ascending address order.

Ben.