Re: [Xen-devel] [PATCH v3 1/8] public / x86: Introduce __HYPERCALL_dm_op...

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 16 January 2017 16:17
> To: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
> Cc: Ian Jackson <Ian.Jackson@xxxxxxxxxx>; Jennifer Herbert
> <jennifer.herbert@xxxxxxxxxx>; Paul Durrant <Paul.Durrant@xxxxxxxxxx>; Wei
> Liu <wei.liu2@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx; Daniel DeGraaf
> <dgdegra@xxxxxxxxxxxxx>
> Subject: Re: [PATCH v3 1/8] public / x86: Introduce __HYPERCALL_dm_op...
> 
> >>> On 16.01.17 at 17:05, <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 13/01/17 12:47, Jan Beulich wrote:
> >>>>>> The kernel already has to parse this structure anyway, and will know
> the
> >>>>>> bitness of its userspace process.  We could easily (at this point)
> >>>>>> require the kernel to turn it into the kernels bitness for forwarding
> on
> >>>>>> to Xen, which covers the 32bit userspace under a 64bit kernel
> problem,
> >>>>>> in a way which won't break the hypercall ABI when 128bit comes
> along.
> >>>> But that won't cover a 32-bit kernel.
> >>> Yes it will.
> >> How that, without a compat translation layer in Xen?
> >
> > Why shouldn't there be a compat layer?
> 
> Because the compat layer we have is kind of ugly to maintain. Hence
> I would expect additions to it to not make the situation any better.
> 
> >>>> And I'm not sure we really need to bother considering hypothetical
> >>>> 128-bit architectures at this point in time.
> >>> Because considering this case will avoid us painting ourselves into a
> >>> corner.
> >> Why would we consider this case here, when all other part of the
> >> public interface don't do so?
> >
> > This is asking why we should continue to shoot ourselves in the foot,
> > ABI wise, rather than trying to do something better.
> >
> > And the answer is that I'd prefer that we started fixing the problem,
> > rather than making it worse.
> 
> Okay, so 128 bit handles then. But wait, we should be prepared for
> 256-bit environments to, so 256-bit handles then. But wait, ...
> 
> Or maybe I'm simply not getting what you mean to put in place here.
> 

<sigh> If we don't want to bake 64-bit pointers into the ABI then I guess a 
compat layer is the only way. Guess I'll go and stare at macros until my brain 
explodes...

  Paul

> Jan


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