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Date:   Thu, 25 Apr 2019 09:07:13 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Zhao Yakui <yakui.zhao@intel.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org, tglx@linutronix.de,
        bp@alien8.de, Jason Chen CJ <jason.cj.chen@intel.com>
Subject: Re: [RFC PATCH v5 4/4] x86/acrn: Add hypercall for ACRN guest
Message-ID: <20190425070712.GA57256@gmail.com>
References: <1556067260-9128-1-git-send-email-yakui.zhao@intel.com>
 <1556067260-9128-5-git-send-email-yakui.zhao@intel.com>
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* Zhao Yakui <yakui.zhao@intel.com> wrote:

> When ACRN hypervisor is detected, the hypercall is needed so that the
> ACRN guest can query/config some settings. For example: it can be used
> to query the resources in hypervisor and manage the CPU/memory/device/
> interrupt for the guest operating system.
> 
> So add the hypercall so that ACRN guest can communicate with the
> low-level ACRN hypervisor. It is implemented with the VMCALL instruction.
> 
> Co-developed-by: Jason Chen CJ <jason.cj.chen@intel.com>
> Signed-off-by: Jason Chen CJ <jason.cj.chen@intel.com>
> Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
> ---
> V1->V2: Refine the comments for the function of acrn_hypercall0/1/2
> v2->v3: Use the "vmcall" mnemonic to replace hard-code byte definition
> v4->v5: Use _ASM_X86_ACRN_HYPERCALL_H instead of _ASM_X86_ACRNHYPERCALL_H to
> align the header file of acrn_hypercall.h
>         Use the "VMCALL" mnemonic in comment/commit log.
>         Uppercase r8/rdi/rsi/rax for hypercall parameter registers in comment.
> ---
>  arch/x86/include/asm/acrn_hypercall.h | 82 +++++++++++++++++++++++++++++++++++
>  1 file changed, 82 insertions(+)
>  create mode 100644 arch/x86/include/asm/acrn_hypercall.h
> 
> diff --git a/arch/x86/include/asm/acrn_hypercall.h b/arch/x86/include/asm/acrn_hypercall.h
> new file mode 100644
> index 0000000..3594436
> --- /dev/null
> +++ b/arch/x86/include/asm/acrn_hypercall.h
> @@ -0,0 +1,82 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _ASM_X86_ACRN_HYPERCALL_H
> +#define _ASM_X86_ACRN_HYPERCALL_H


> +
> +#include <linux/errno.h>
> +
> +#ifdef CONFIG_ACRN_GUEST
> +
> +/*
> + * Hypercalls for ACRN guest
> + *
> + * Hypercall number is passed in R8 register.
> + * Up to 2 arguments are passed in RDI, RSI.
> + * Return value will be placed in RAX.
> + */
> +
> +static inline long acrn_hypercall0(unsigned long hcall_id)
> +{
> +	register unsigned long r8 asm("r8") = hcall_id;
> +	register long result asm("rax");
> +
> +	/* the hypercall is implemented with the VMCALL instruction.
> +	 * asm indicates that inline assembler instruction is used.
> +	 * volatile qualifier is added to avoid that it is dropped
> +	 * because of compiler optimization.
> +	 */

Non-standard comment style.

asm statements are volatile by default I believe.

I.e. the second and third sentences are partly obvious, superfluous and 
bogus.

> +	asm volatile("vmcall"
> +			: "=r"(result)
> +			: "r"(r8));
> +
> +	return result;
> +}
> +
> +static inline long acrn_hypercall1(unsigned long hcall_id,
> +				   unsigned long param1)
> +{
> +	register unsigned long r8 asm("r8") = hcall_id;
> +	register long result asm("rax");
> +
> +	asm volatile("vmcall"
> +			: "=r"(result)
> +			: "D"(param1), "r"(r8));

Why are register variables used? Doesn't GCC figure it out correctly by 
default?

> +static inline long acrn_hypercall2(unsigned long hcall_id,
> +				   unsigned long param1,
> +				   unsigned long param2)
> +{
> +	register unsigned long r8 asm("r8") = hcall_id;
> +	register long result asm("rax");
> +
> +	asm volatile("vmcall"
> +			: "=r"(result)
> +			: "D"(param1), "S"(param2), "r"(r8));

Ditto.

Thanks,

	Ingo