Re: [Xen-devel] [PATCH] Enble 6 argument hypercalls for HVMs

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> On 15.12.10 at 11:06, Keir Fraser <keir@xxxxxxx> wrote:
> On 15/12/2010 09:07, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:
> 
>>>>> On 14.12.10 at 23:16, Ross Philipson <Ross.Philipson@xxxxxxxxxx> wrote:
>>> Enable 6 argument hypercalls for HVMs. The hypercall code handles a sixth
>>> argument in EBP or R9 but the HVM code is not passing the value.
>>> 
>>> Signed-off-by: Ross Philipson <ross.philipson@xxxxxxxxxx>
>> 
>> I'm curious what hypercall there is that takes 6 arguments,
>> particularly on 64-bit (where the maximum so far is 4).
> 
> The v4v hypercalls in XenClient (not as yet submitted upstream) take 6
> arguments. Multicalls also need fixing up for a sixth argument, making
> everything consistent with existing PV hypercall logic.

I would generally take this as an indication that this actually works,
but at least with tracing enabled I can't see how it would on 64-bit
(note the last two reloads):

        call  trace_hypercall
        /* Now restore all the registers that trace_hypercall clobbered */
        movq  UREGS_rax+SHADOW_BYTES(%rsp),%rax   /* Hypercall #  */
        movq  UREGS_rdi+SHADOW_BYTES(%rsp),%rdi   /* Arg 1        */
        movq  UREGS_rsi+SHADOW_BYTES(%rsp),%rsi   /* Arg 2        */
        movq  UREGS_rdx+SHADOW_BYTES(%rsp),%rdx   /* Arg 3        */
        movq  UREGS_r10+SHADOW_BYTES(%rsp),%rcx   /* Arg 4        */
        movq  UREGS_rdi+SHADOW_BYTES(%rsp),%r8    /* Arg 5        */
        movq  UREGS_rbp+SHADOW_BYTES(%rsp),%r9    /* Arg 6        */

Looking at this code also makes me wonder once again whether
it really is a good idea to have a generally not taken forward
branch here.

Jan


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