Re: [Xen-devel] [PATCH 1/9] x86/hypercall: Move some of the hvm hypercall infrastructure into hypercall.h

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 2 Aug 2016, Jan Beulich wrote:
> >>> On 02.08.16 at 16:59, <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 02/08/16 15:54, Jan Beulich wrote:
> >>>>> On 02.08.16 at 16:26, <julien.grall@xxxxxxx> wrote:
> >>> On 02/08/16 15:17, Jan Beulich wrote:
> >>>> Well, I find it quite odd for hypercall argument counts to differ
> >>>> between arches. I.e. I'd conclude the ABI was mis-specified.
> >>> Is it documented somewhere for the x86 code? Looking at Linux, the 
> >>> privcmd call is only passing 5 arguments on both ARM and x86.
> >> arch-x86/xen-x86_32.h has
> >>
> >>  * Hypercall interface:
> >>  *  Input:  %ebx, %ecx, %edx, %esi, %edi, %ebp (arguments 1-6)
> >>  *  Output: %eax
> >>
> >> while arch-x86/xen-x86_64.h has
> >>
> >>  * Hypercall interface:
> >>  *  Input:  %rdi, %rsi, %rdx, %r10, %r8, %r9 (arguments 1-6)
> >>  *  Output: %rax
> > 
> > The only actual 6 argument hypercall is the v4v hypercall, better known
> > as __HYPERVISOR_xc_reserved_op at index 39, but that isn't implemented
> > anywhere upstream.
> 
> But it serves as an example what now wouldn't work on ARM.

At the time the arm hypercall ABI was published, it matched the x86
hypercall ABI, which had only 5 hypercall arguments.

The issue is that the x86 hypercall ABI changed, and now is out of sync
with ARM. The faulty commit being:

commit 4af64160c580b02f28c992c09d55957cb20a9b91
Author: David Vrabel <david.vrabel@xxxxxxxxxx>
Date:   Wed May 30 09:25:11 2012 +0100

    x86: document register for 6th hypercall argument
    
    From: David Vrabel <david.vrabel@xxxxxxxxxx>
    
    Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
    Committed-by: Keir Fraser <keir@xxxxxxx>


While the ARM ABI is from few months earlier:

commit 40f20c4bfcd5d25c90f9419250ca8a229bf4c1e5
Author: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Date:   Tue Mar 13 16:04:05 2012 +0000

    arm: use r12 to pass the hypercall number
    
    ** This is a guest visible ABI change which requires an updated guest 
kernel **
    
    Use r12 to pass the hypercall number and r0-r4 for the hypercall
    arguments.
    
    Use the ISS to pass an hypervisor specific tag.
    
    Remove passing unused registers to arm_hypercall_table: we don't have 6
    arguments hypercalls and we never use 64 bit values as hypercall
    arguments, 64 bit values are only contained within structs passed as
    arguments.
    
    Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
    [ use #ifndef NDEBUG, fix coding style, expand calling convention comment
      slightly and added a big fat note about ABI change - ijc ]
 

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel