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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 3 Aug 2016, Julien Grall wrote:
> Hi Jan,
> 
> On 03/08/16 09:53, Jan Beulich wrote:
> > > > > On 02.08.16 at 20:43, <sstabellini@xxxxxxxxxx> wrote:
> > > 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:
> > 
> > That's one way of viewing it, but I don't think an appropriate one.
> > 6-argument hypercalls had always been possible on x86, just that
> > they might not have been documented in the public headers (but
> > instead only in the actual hypercall implementation).
> 
> I would tend to say that anything not documented in the public header is not
> part of the ABI regardless how it has been implemented before hand.

I agree. What is documented (or not documented) in the public headers
is the golden standard.


> Anyway, I looked at the hypercall implementation on ARM and it seems that we
> half support the 6th argument. For instance hypercall_create_continuation is
> clobbering r5/x5 which is not part of the ABI.
> 
> However do_trap_hypercall is only supporting up to 5 argument.
> 
> I don't think it would be an issue to support 6 arguments on ARM. Stefano,
> what do you think?

I am OK with supporting 6 arguments hypercalls and it would be good to
have both architectures match. The 32bit guest-side implementation needs
special caring, but it should be OK (see HYPERCALL5 in Linux).
Fortunately xen/include/public/arch-arm.h states that only arguments
used by an hypercall can be clobbered.

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