x86/sev: Expose sev_es_ghcb_hv_call() for use by HyperV

Hyper-V needs to issue the GHCB HV call in order to read/write MSRs in
Isolation VMs. For that, expose sev_es_ghcb_hv_call().

The Hyper-V Isolation VMs are unenlightened guests and run a paravisor
at VMPL0 for communicating. GHCB pages are being allocated and set up
by that paravisor. Linux gets the GHCB page's physical address via
MSR_AMD64_SEV_ES_GHCB from the paravisor and should not change it.

Add a @set_ghcb_msr parameter to sev_es_ghcb_hv_call() to control
whether the function should set the GHCB's address prior to the call or
not and export that function for use by HyperV.

  [ bp: - Massage commit message
        - add a struct ghcb forward declaration to fix randconfig builds. ]

Signed-off-by: Tianyu Lan <Tianyu.Lan@microsoft.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20211025122116.264793-6-ltykernel@gmail.com

1 files changed, 16 insertions, 9 deletions

diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
index 4579c38a11c4..0aacd6047ef2 100644
--- a/arch/x86/kernel/sev-shared.c
+++ b/arch/x86/kernel/sev-shared.c
@@ -125,10 +125,9 @@ 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,
-					  struct es_em_ctxt *ctxt,
-					  u64 exit_code, u64 exit_info_1,
-					  u64 exit_info_2)
+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)
 {
 	/* Fill in protocol and format specifiers */
 	ghcb->protocol_version = GHCB_PROTOCOL_MAX;
@@ -138,7 +137,14 @@ 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 and set up by that paravisor. Linux
+	 * should not change the GHCB page's physical address.
+	 */
+	if (set_ghcb_msr)
+		sev_es_wr_ghcb_msr(__pa(ghcb));
+
 	VMGEXIT();
 
 	return verify_exception_info(ghcb, ctxt);
@@ -418,7 +424,7 @@ static enum es_result vc_handle_ioio(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
 		 */
 		sw_scratch = __pa(ghcb) + offsetof(struct ghcb, shared_buffer);
 		ghcb_set_sw_scratch(ghcb, sw_scratch);
-		ret = sev_es_ghcb_hv_call(ghcb, ctxt, SVM_EXIT_IOIO,
+		ret = sev_es_ghcb_hv_call(ghcb, true, ctxt, SVM_EXIT_IOIO,
 					  exit_info_1, exit_info_2);
 		if (ret != ES_OK)
 			return ret;
@@ -460,7 +466,8 @@ static enum es_result vc_handle_ioio(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
 
 		ghcb_set_rax(ghcb, rax);
 
-		ret = sev_es_ghcb_hv_call(ghcb, ctxt, SVM_EXIT_IOIO, exit_info_1, 0);
+		ret = sev_es_ghcb_hv_call(ghcb, true, ctxt,
+					  SVM_EXIT_IOIO, exit_info_1, 0);
 		if (ret != ES_OK)
 			return ret;
 
@@ -491,7 +498,7 @@ static enum es_result vc_handle_cpuid(struct ghcb *ghcb,
 		/* xgetbv will cause #GP - use reset value for xcr0 */
 		ghcb_set_xcr0(ghcb, 1);
 
-	ret = sev_es_ghcb_hv_call(ghcb, ctxt, SVM_EXIT_CPUID, 0, 0);
+	ret = sev_es_ghcb_hv_call(ghcb, true, ctxt, SVM_EXIT_CPUID, 0, 0);
 	if (ret != ES_OK)
 		return ret;
 
@@ -516,7 +523,7 @@ static enum es_result vc_handle_rdtsc(struct ghcb *ghcb,
 	bool rdtscp = (exit_code == SVM_EXIT_RDTSCP);
 	enum es_result ret;
 
-	ret = sev_es_ghcb_hv_call(ghcb, ctxt, exit_code, 0, 0);
+	ret = sev_es_ghcb_hv_call(ghcb, true, ctxt, exit_code, 0, 0);
 	if (ret != ES_OK)
 		return ret;