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

[Julien Grall <julien@xen.org>]

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.

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.

Cheers,

-- 
Julien Grall