linux-kernel - Re: [RFC PATCH V6 08/14] x86/hyperv: Use vmmcall to implement Hyper-V hypercall in sev-snp enlightened guest

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Message-ID: <20230516102912.GG2587705@hirez.programming.kicks-ass.net>
Date:   Tue, 16 May 2023 12:29:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tianyu Lan <ltykernel@...il.com>
Cc:     luto@...nel.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, seanjc@...gle.com, pbonzini@...hat.com,
        jgross@...e.com, tiala@...rosoft.com, kirill@...temov.name,
        jiangshan.ljs@...group.com, ashish.kalra@....com,
        srutherford@...gle.com, akpm@...ux-foundation.org,
        anshuman.khandual@....com, pawan.kumar.gupta@...ux.intel.com,
        adrian.hunter@...el.com, daniel.sneddon@...ux.intel.com,
        alexander.shishkin@...ux.intel.com, sandipan.das@....com,
        ray.huang@....com, brijesh.singh@....com, michael.roth@....com,
        thomas.lendacky@....com, venu.busireddy@...cle.com,
        sterritt@...gle.com, tony.luck@...el.com, samitolvanen@...gle.com,
        fenghua.yu@...el.com, pangupta@....com,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-hyperv@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [RFC PATCH V6 08/14] x86/hyperv: Use vmmcall to implement
 Hyper-V hypercall in sev-snp enlightened guest

On Mon, May 15, 2023 at 12:59:10PM -0400, Tianyu Lan wrote:
> From: Tianyu Lan <tiala@...rosoft.com>
> 
> In sev-snp enlightened guest, Hyper-V hypercall needs
> to use vmmcall to trigger vmexit and notify hypervisor
> to handle hypercall request.
> 
> Signed-off-by: Tianyu Lan <tiala@...rosoft.com>
> ---
> Change since RFC V2:
>        * Fix indentation style
> ---
>  arch/x86/include/asm/mshyperv.h | 44 ++++++++++++++++++++++++---------
>  1 file changed, 33 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
> index 97d117ec95c4..939373791249 100644
> --- a/arch/x86/include/asm/mshyperv.h
> +++ b/arch/x86/include/asm/mshyperv.h
> @@ -61,16 +61,25 @@ static inline u64 hv_do_hypercall(u64 control, void *input, void *output)
>  	u64 hv_status;
>  
>  #ifdef CONFIG_X86_64
> -	if (!hv_hypercall_pg)
> -		return U64_MAX;
> +	if (hv_isolation_type_en_snp()) {
> +		__asm__ __volatile__("mov %4, %%r8\n"
> +				     "vmmcall"
> +				     : "=a" (hv_status), ASM_CALL_CONSTRAINT,
> +				       "+c" (control), "+d" (input_address)
> +				     :  "r" (output_address)
> +				     : "cc", "memory", "r8", "r9", "r10", "r11");
> +	} else {
> +		if (!hv_hypercall_pg)
> +			return U64_MAX;
>  
> -	__asm__ __volatile__("mov %4, %%r8\n"
> -			     CALL_NOSPEC
> -			     : "=a" (hv_status), ASM_CALL_CONSTRAINT,
> -			       "+c" (control), "+d" (input_address)
> -			     :  "r" (output_address),
> -				THUNK_TARGET(hv_hypercall_pg)
> -			     : "cc", "memory", "r8", "r9", "r10", "r11");
> +		__asm__ __volatile__("mov %4, %%r8\n"
> +				     CALL_NOSPEC
> +				     : "=a" (hv_status), ASM_CALL_CONSTRAINT,
> +				       "+c" (control), "+d" (input_address)
> +				     :  "r" (output_address),
> +					THUNK_TARGET(hv_hypercall_pg)
> +				     : "cc", "memory", "r8", "r9", "r10", "r11");
> +	}

Wouldn't this generate better code with an alternative?