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Re: [Qemu-devel] [qemu-s390x] [PATCH v3 1/7] s390x/pci: factor out endia

From: Cornelia Huck
Subject: Re: [Qemu-devel] [qemu-s390x] [PATCH v3 1/7] s390x/pci: factor out endianess conversion
Date: Mon, 27 Nov 2017 12:02:55 +0100

On Mon, 27 Nov 2017 07:59:36 +0100
Thomas Huth <address@hidden> wrote:

> On 25.11.2017 14:49, Pierre Morel wrote:
> > On 24/11/2017 07:19, Yi Min Zhao wrote:  
> >>
> >>
> >> 在 2017/11/23 下午8:18, Thomas Huth 写道:  
> >>> On 23.11.2017 13:07, Yi Min Zhao wrote:  

> >>>> Another question, does 'cpu' in cpu_to_le**() or le**_to_cpu() mean the
> >>>> host endianess?  
> >>> Yes, the "cpu" in cpu_to_le or le_to_cpu means the host, indeed. It's
> >>> confusing :-/
> >>>  
> >>>> If the answers to upper two questions are yes, we actually need handle
> >>>> two cases.
> >>>> 1) For pcilg, we need to translate the data to little-endian, thus
> >>>> cpu_to_le**().
> >>>> 2) For pcistg, we need to translate the data to host endianess, thus
> >>>> le**_to_cpu().  
> >>> I think we've got to byte-swap if the host is big endian (s390x), but
> >>> not if the host is little endian (x86 with TCG).  
> > 
> > Here is my comprehension of this funny swapping:
> > 
> > - TCG for a BE guest and a le host swap bytes because if we do (register
> > & 0x01) in the zPCI interception code it must work what ever the
> > endianess is.  
> Uhhh, I might have missed that the value has already been byte-swapped
> once by TCG for env->regs[r1] ...
> Now I'm pretty much completely confused ... sorry for the noise if I was
> wrong... I think it's best you ignore my comment for now (i.e. go with
> bswapXX() instead of le_to_cpuXX()), and if we later wire up zPCI with
> TCG, we still can fix this if necessary.

I'll try my current pci/tcg patches on LPAR with this (or a v4) on top.
If it works there (it doesn't yet on my laptop), we do have a
endianness issue... (unfortunately, the reverse isn't true.)

I'll post my pci/tcg patches once I get it mostly working (and it does
not look like a horror from the crypt anymore).

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