linux-kernel - Re: [PATCH v5 04/17] x86/acrn: Introduce hypercall interfaces

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Date:   Tue, 20 Oct 2020 10:16:02 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Shuo A Liu <shuo.a.liu@...el.com>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Yu Wang <yu1.wang@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Yakui Zhao <yakui.zhao@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Fengwei Yin <fengwei.yin@...el.com>,
        Zhi Wang <zhi.a.wang@...el.com>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Segher Boessenkool <segher@...nel.crashing.org>
Subject: Re: [PATCH v5 04/17] x86/acrn: Introduce hypercall interfaces

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@...el.com wrote:
> >> >> From: Shuo Liu <shuo.a.liu@...el.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?