Re: [Xen-devel] [PATCH 4/5] xen: introduce XEN_GUEST_HANDLE_PARAM

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 06.08.12 at 17:47, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Mon, 2012-08-06 at 16:43 +0100, Jan Beulich wrote:
>> >>> On 06.08.12 at 16:12, Stefano Stabellini 
>> >>> <stefano.stabellini@xxxxxxxxxxxxx> 
> wrote:
>> > Note: this change does not make any difference on x86 and ia64.
>> > 
>> > 
>> > XEN_GUEST_HANDLE_PARAM is going to be used to distinguish guest pointers
>> > stored in memory from guest pointers as hypercall parameters.
>> 
>> I have to admit that really dislike this, to a large part because of
>> the follow up patch that clutters the corresponding function
>> declarations even further. Plus I see no mechanism to convert
>> between the two, yet I can't see how - long term at least - you
>> could get away without such conversion.
>> 
>> Is it really a well thought through and settled upon decision to
>> make guest handles 64 bits wide even on 32-bit ARM? After all,
>> both x86 and PPC got away without doing so
> 
> Well, on x86 we have the compat XLAT layer, which is a pretty complex
> piece of code, so "got away without" is a bit strong...

Hmm, yes, that's a valid correction.

> We'd really
> rather not have to have a non-trivial compat layer on arm too by having
> the struct layouts be the same on 32/64.

And paying a penalty like this in the 32-bit half (if what is likely
to remain the much bigger portion for the next couple of years
can validly be called "half") is worth it? The more that the compat
layer is now reasonably mature (and should hence be easily
re-usable for ARM)?

Jan


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