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Subject: Re: [PATCH] x86/sev-es: Expose __sev_es_ghcb_hv_call() to call ghcb
 hv call out of sev code
To:     Borislav Petkov <bp@alien8.de>, Tianyu Lan <ltykernel@gmail.com>
Cc:     Tianyu Lan <Tianyu.Lan@microsoft.com>, kys@microsoft.com,
        haiyangz@microsoft.com, sthemmin@microsoft.com, wei.liu@kernel.org,
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References: <2772390d-09c1-80c1-082f-225f32eae4aa@gmail.com>
 <20211020062321.3581158-1-ltykernel@gmail.com> <YW/oaZ2GN15hQdyd@zn.tnic>
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From:   Tom Lendacky <thomas.lendacky@amd.com>
Message-ID: <62ffaeb4-1940-4934-2c39-b8283d402924@amd.com>
Date:   Wed, 20 Oct 2021 08:56:22 -0500
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On 10/20/21 8:39 AM, Borislav Petkov wrote:
> On Wed, Oct 20, 2021 at 08:39:59PM +0800, Tianyu Lan wrote:
>> Hyper-V runs paravisor in guest VMPL0 which emulates some functions
>> (e.g, timer, tsc, serial console and so on) via handling VC exception.
>> GHCB pages are allocated and set up by the paravisor and report to Linux
>> guest via MSR register.Hyper-V SEV implementation is unenlightened guest
>> case which doesn't Linux doesn't handle VC and paravisor in the VMPL0
>> handle it.
> 
> Aha, unenlightened.
> 
> So why don't you export the original function by doing this (only
> partial diff to show intent only):
> 
> ---
> diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
> index f1d513897baf..bfe82f58508f 100644
> --- a/arch/x86/kernel/sev-shared.c
> +++ b/arch/x86/kernel/sev-shared.c
> @@ -125,7 +125,7 @@ static enum es_result verify_exception_info(struct ghcb *ghcb, struct es_em_ctxt
>   	return ES_VMM_ERROR;
>   }
>   
> -static enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb,
> +static enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb, bool set_ghcb_msr,
>   					  struct es_em_ctxt *ctxt,
>   					  u64 exit_code, u64 exit_info_1,
>   					  u64 exit_info_2)
> @@ -138,7 +138,14 @@ static enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb,
>   	ghcb_set_sw_exit_info_1(ghcb, exit_info_1);
>   	ghcb_set_sw_exit_info_2(ghcb, exit_info_2);
>   
> -	sev_es_wr_ghcb_msr(__pa(ghcb));
> +	/*
> +	 * Hyper-V unenlightened guests use a paravisor for communicating and
> +	 * GHCB pages are being allocated by that paravisor which uses a
> +	 * different MSR and protocol.

Just to clarify the comment, the paravisor uses the same GHCB MSR and GHCB 
protocol, it just can't use __pa() to get the address of the GHCB. So I 
expect that the Hyper-V support sets the address properly before calling 
this function.

Thanks,
Tom

> +	 */
> +	if (set_ghcb_msr)
> +		sev_es_wr_ghcb_msr(__pa(ghcb));
> +
>   	VMGEXIT();
>   
>   	return verify_exception_info(ghcb, ctxt);
> 
>