Re: [PATCH v2 4/4] x86: adjustments to guest handle treatment

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Apr 21, 2020 at 07:44:55PM +0100, Julien Grall wrote:
> Hi,
> 
> On 21/04/2020 18:30, Roger Pau Monné wrote:
> > On Tue, Apr 21, 2020 at 11:13:23AM +0200, Jan Beulich wrote:
> > > First of all avoid excessive conversions. copy_{from,to}_guest(), for
> > > example, work fine with all of XEN_GUEST_HANDLE{,_64,_PARAM}().
> > 
> > I'm not sure I understand the difference between those two, as they
> > are both placeholders for linear guest addresses?
> > 
> > AFAICT XEN_GUEST_HANDLE should be used for guest pointers inside of an
> > hypercall struct, while XEN_GUEST_HANDLE_PARAM is for guest pointers
> > as hypercall arguments. But those are both just guest pointers,
> > whether they are a parameter to the hypercall or a field in a
> > struct, and hence could use the same type?
> > 
> > I assume there's some reason for not doing so, and I see the comment
> > about other arches, but again a linear guest address is just that in
> > all arches, regardless of it's placement.
> 
> On Arm:
>  * XEN_GUEST_HANDLE() will always be 64-bit on both 32-bit and 64-bit
> hypervisor.
>  * XEN_GUEST_HANDLE_PARAM() will be 32-bit for 32-bit hypervisor. For 64-bit
> hypervisor, it will be 64-bit.
> 
> Per the ABI, each argument only fit a register. So you could not use
> XEN_GUEST_HANDLE() as now an argument will be held in 2 registers on 32-bit.
> 
> We also want the structure layout to be the same for 32-bit and 64-bit. So
> using XEN_GUEST_HANDLE_PARAM() everywhere is not the solution as the virtual
> address is not the same.

Right, you hide the 'padding' inside XEN_GUEST_HANDLE by making it
have a fixed size on all bitnesses, I can see how that's not
desirable for hypercall params though.

Iff we ever switch to an ABI that uses physical addresses instead of
linear ones we would have to switch everything to be a 64bit integer,
or else 32bit PAE won't work correctly. Or come up with a completely
different ABI (ie: use a pre-allocated set of buffer pages, IIRC as
suggested by Juergen).

> 
> We could possibly convert internally XEN_GUEST_HANDLE_PARAM() to
> XEN_GUEST_HANDLE(), but I am not sure how this would be beneficial. We would
> have to use 2 registers for arm 32-bit everytime.

Hm, we could maybe expand hypercall parameters to 64bit using some
kind of translation layer between the entry point and the actual
handler, but anyway, I get now there's a need to keep this difference.

Thanks, Roger.