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

From: Borislav Petkov <[email-masked]>
Date: Wed, 20 Oct 2021 15:39:38 +0200

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

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette