linux-kernel - Re: [PATCH 5/9] x86/hyperv: Use vmmcall to implement Hyper-V hypercall in sev-snp enlightened guest

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Message-ID: <20230608132127.GK998233@hirez.programming.kicks-ass.net>
Date:   Thu, 8 Jun 2023 15:21:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tianyu Lan <ltykernel@...il.com>
Cc:     kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
        decui@...rosoft.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, daniel.lezcano@...aro.org, arnd@...db.de,
        michael.h.kelley@...rosoft.com, Tianyu Lan <tiala@...rosoft.com>,
        linux-arch@...r.kernel.org, linux-hyperv@...r.kernel.org,
        linux-kernel@...r.kernel.org, vkuznets@...hat.com
Subject: Re: [PATCH 5/9] x86/hyperv: Use vmmcall to implement Hyper-V
 hypercall in sev-snp enlightened guest

On Thu, Jun 01, 2023 at 11:16:18AM -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.
> 
> There is no x86 SEV SNP feature flag support so far and
> hardware provides MSR_AMD64_SEV register to check SEV-SNP
> capability with MSR_AMD64_SEV_ENABLED bit. ALTERNATIVE can't
> work without SEV-SNP x86 feature flag. May add later when
> the associated flag is introduced. 
> 
> Signed-off-by: Tianyu Lan <tiala@...rosoft.com>
> ---
>  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 31c476f4e656..d859d7c5f5e8 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");
> +	}
>  #else

Remains unanswered:

https://lkml.kernel.org/r/20230516102912.GG2587705%40hirez.programming.kicks-ass.net

Would this not generate better code with an alternative?