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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Jan 13, 2017 at 09:05:19AM +0000, Paul Durrant wrote:
> > -----Original Message-----
> > From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> > Sent: 12 January 2017 16:29
> > To: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Paul Durrant
> > <Paul.Durrant@xxxxxxxxxx>
> > Cc: Ian Jackson <Ian.Jackson@xxxxxxxxxx>; Jennifer Herbert
> > <jennifer.herbert@xxxxxxxxxx>; Wei Liu <wei.liu2@xxxxxxxxxx>; xen-
> > devel@xxxxxxxxxxxxxxxxxxxx; Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>
> > Subject: RE: [PATCH v3 1/8] public / x86: Introduce __HYPERCALL_dm_op...
> > 
> > >>> On 12.01.17 at 17:10, <Paul.Durrant@xxxxxxxxxx> wrote:
> > >> > +
> > >> > +struct xen_dm_op_buf {
> > >> > +    XEN_GUEST_HANDLE_64(void) h;
> > >> > +    uint32_t size;
> > >> > +};
> > >>
> > >> Sorry to quibble, but there is a problem here which has only just
> > >> occurred to me.  This ABI isn't futureproof, and has padding at the end
> > >> which affects how the array is layed out.
> > 
> > Yes, padding needs to be added.
> > 
> > >> The userspace side should be
> > >>
> > >> struct xen_dm_op_buf {
> > >>     void *h;
> > >>     size_t size;
> > >> }
> > >>
> > >> which will work sensibly for 32bit and 64bit userspace, and futureproof
> > >> (for when 128bit turns up).  Its size is also a power of two which
> > >> avoids alignment issues in the array.
> > >>
> > >> 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.
> >
> 
> Do we need to care about a 32-bit kernel for a tools-only hypercall? I 
> thought a toolstack already had to be (at least) 64-bit to match Xen.
> 

Toolstack can run on 32bit kernel on 64bit Xen.

Wei.

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