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Sender: Arvind Sankar <niveditas98@gmail.com>
From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Tue, 20 Oct 2020 10:16:02 -0400
To:     Shuo A Liu <shuo.a.liu@intel.com>
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Subject: Re: [PATCH v5 04/17] x86/acrn: Introduce hypercall interfaces
Message-ID: <20201020141602.GD2996696@rani.riverdale.lan>
References: <20201019061803.13298-1-shuo.a.liu@intel.com>
 <20201019061803.13298-5-shuo.a.liu@intel.com>
 <20201019221515.GA2875488@rani.riverdale.lan>
 <20201020013809.GA11038@shuo-intel.sh.intel.com>
 <20201020020851.GA2996696@rani.riverdale.lan>
 <20201020023017.GA12408@shuo-intel.sh.intel.com>
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In-Reply-To: <20201020023017.GA12408@shuo-intel.sh.intel.com>
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On Tue, Oct 20, 2020 at 10:30:17AM +0800, Shuo A Liu wrote:
> On Mon 19.Oct'20 at 22:08:51 -0400, Arvind Sankar wrote:
> >On Tue, Oct 20, 2020 at 09:38:09AM +0800, Shuo A Liu wrote:
> >> On Mon 19.Oct'20 at 18:15:15 -0400, Arvind Sankar wrote:
> >> >On Mon, Oct 19, 2020 at 02:17:50PM +0800, shuo.a.liu@intel.com wrote:
> >> >> From: Shuo Liu <shuo.a.liu@intel.com>
> >> >>
> >> >> The Service VM communicates with the hypervisor via conventional
> >> >> hypercalls. VMCALL instruction is used to make the hypercalls.
> >> >>
> >> >> ACRN hypercall ABI:
> >> >>   * Hypercall number is in R8 register.
> >> >>   * Up to 2 parameters are in RDI and RSI registers.
> >> >>   * Return value is in RAX register.
> >> >>
> >> >> Introduce the ACRN hypercall interfaces. Because GCC doesn't support R8
> >> >> register as direct register constraints, use supported constraint as
> >> >> input with a explicit MOV to R8 in beginning of asm.
> >> >>
> >> >> +static inline long acrn_hypercall0(unsigned long hcall_id)
> >> >> +{
> >> >> +	long result;
> >> >> +
> >> >> +	asm volatile("movl %1, %%r8d\n\t"
> >> >> +		     "vmcall\n\t"
> >> >> +		     : "=a" (result)
> >> >> +		     : "ir" (hcall_id)
> >> >
> >> >Is the hypercall id an unsigned long (64 bits) or an unsigned int (32
> >> >bits)? This will generate broken assembly if the "r" option is chosen,
> >> >eg something like
> >> >	movl %rdi, %r8d
> >>
> >> Yes, it can be an unsigned long. So do MOV explicitly.
> >>
> >> 	asm volatile("movq %1, %%r8\n\t"
> >> 		     "vmcall\n\t"
> >> 		     : "=a" (result)
> >> 		     : "ir" (hcall_id)
> >>
> >> Thanks
> >
> >All the hypercall ID's defined seem to be only 32 bits though?
> 
> Yes, they are.
> The paramter is unsigned long, use movq to align it.

I don't understand what you mean by alignment here, but I was asking why
hcall_id is unsigned long and not unsigned int (or u32) if you only need
32 bits?