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

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