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

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Message-ID: <20201021011606.GB12408@shuo-intel.sh.intel.com>
Date:   Wed, 21 Oct 2020 09:16:06 +0800
From:   Shuo A Liu <shuo.a.liu@...el.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     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>,
        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 20.Oct'20 at 10:16:02 -0400, Arvind Sankar wrote:
>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?

The hypervisor is using R8 as the input of hcall_id. So i just want to
keep it consistent.