linux-kernel - Re: ibmvtpm byteswapping inconsistency

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Date:   Sat, 28 Jan 2017 06:58:57 +1100
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Tyrel Datwyler <tyreld@...ux.vnet.ibm.com>,
        Michal Suchánek <msuchanek@...e.de>,
        Ashley Lai <ashleydlai@...il.com>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Peter Huewe <peterhuewe@....de>,
        Marcel Selhorst <tpmdd@...horst.net>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        tpmdd-devel@...ts.sourceforge.net, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: ibmvtpm byteswapping inconsistency

On Fri, 2017-01-27 at 10:02 -0800, Tyrel Datwyler wrote:
> > The problem is that we are packing an in-memory structure into 2
> > registers and it's expected that this structure is laid out in the
> > registers as if it had been loaded by a BE CPU.
> 
> This is only the case if the cpu is BE. If the cpu is LE, regardless of
> the fact that our in memory structure is laid out BE, when we break it
> into 2 words each of those words needs to be loaded LE.

That doesn't make sense and doesn't match the code... The structure
needs to always have the same in-register layout regardless of the
endianness of the CPU, especially since the underlying hypervisor
will most likely be BE :-)

Thta's why the code does a be64_to_cpu() when loading it, this in
effect performs a "BE" load, which on a BE CPU is just a normal load
and on LE is a swap to compensate for the CPU loading it the "wrong way
around".

Cheers,
Ben.