Re: [PATCH v4 1/9] libxc: add DECLARE_HYPERCALL_BUFFER_SHADOW()

[David Vrabel <david.vrabel@citrix.com>]

On 07/05/14 12:45, Ian Campbell wrote:
> On Wed, 2014-04-30 at 19:36 +0100, Andrew Cooper wrote:
>> From: David Vrabel <david.vrabel@citrix.com>
>>
>> DECLARE_HYPERCALL_BUFFER_SHADOW() is like DECLARE_HYPERCALL_BUFFER()
>> except it is backed by an already allocated hypercall buffer.
> 
> I suppose enhancing DECLARE_HYPERCALL_BUFFER_ARGUMENT to have this
> property has issues with unused variables?

Yes, I think so. Although I didn't actually try it... Perhaps I should.

> HYPERCALL_BUFFER_AS_PTR() would have been an alternative implementation
> (similar to AS_ARG), I suppose there is no particular reason to prefer
> one over the other?

Usage of the buffer wouldn't be consistent with a regular
DECLARE_HYPERCALL_BUFFER().  It would lead to slightly more confusing
code like:

DECLARE_HYPERCALL_BUFFER(hbuf);
unsigned long *to_send = HYPERCALL_BUFFER_AS_PTR(hbuf);
...
    if ( test_bit(p, to_send) )
...
    rc = xc_shadow_control(xch, ctx->domid,
                      XEN_DOMCTL_SHADOW_OP_CLEAN,
                      hbuf,
                      ctx->save.p2m_size
                      NULL, 0, shadow_stats);

And it's not as obvious that hbuf and to_send are really the same buffer
without going back to the declarations.

David