netdev - Re: [PATCH] x86/sev-es: Expose __sev_es_ghcb_hv_call() to call ghcb hv call out of sev code

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Message-ID: <62ffaeb4-1940-4934-2c39-b8283d402924@amd.com>
Date:   Wed, 20 Oct 2021 08:56:22 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Borislav Petkov <bp@...en8.de>, Tianyu Lan <ltykernel@...il.com>
Cc:     Tianyu Lan <Tianyu.Lan@...rosoft.com>, kys@...rosoft.com,
        haiyangz@...rosoft.com, sthemmin@...rosoft.com, wei.liu@...nel.org,
        decui@...rosoft.com, tglx@...utronix.de, mingo@...hat.com,
        x86@...nel.org, hpa@...or.com, dave.hansen@...ux.intel.com,
        luto@...nel.org, peterz@...radead.org, davem@...emloft.net,
        kuba@...nel.org, gregkh@...uxfoundation.org, arnd@...db.de,
        jroedel@...e.de, brijesh.singh@....com, pgonda@...gle.com,
        akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
        rppt@...nel.org, tj@...nel.org, aneesh.kumar@...ux.ibm.com,
        saravanand@...com, hannes@...xchg.org, rientjes@...gle.com,
        michael.h.kelley@...rosoft.com, linux-arch@...r.kernel.org,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, vkuznets@...hat.com,
        konrad.wilk@...cle.com, hch@....de, robin.murphy@....com,
        joro@...tes.org, parri.andrea@...il.com, dave.hansen@...el.com
Subject: Re: [PATCH] x86/sev-es: Expose __sev_es_ghcb_hv_call() to call ghcb
 hv call out of sev code

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);
> 
>