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

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Message-ID: <20201020013809.GA11038@shuo-intel.sh.intel.com>
Date:   Tue, 20 Oct 2020 09:38:09 +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>,
        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 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