[PATCH v3 7/7] mshv: set up own SynIC registers on a nested root partition

[Wei Hu <weh@linux.microsoft.com>]

From: Wei Hu <weh@microsoft.com>

Upstream mshv_synic_cpu_init()/_exit() skip programming the SynIC
SIMP/SIEFP/SCONTROL registers when VMBus is active (hv_vmbus_exists()),
assuming VMBus's hv_hyp_synic_enable_regs() already provisioned them.

That assumption is wrong for a *nested* root partition. There, VMBus
programs the *nested* SynIC MSRs (HV_X64_MSR_NESTED_SIMP, ...) via
hv_set_msr()'s nested remap, while mshv_synic reads/writes the
*non-nested* SynIC MSRs (hv_get/set_non_nested_msr). As a result mshv
maps the wrong message page, never receives the SYNIC intercept message
that carries the async hypercall completion, and
mshv_async_hvcall_handler blocks forever -- hanging SEV-SNP guest
creation from user space.

Gate the "VMBus owns the SynIC registers" optimization on !hv_nested so
that a nested root partition programs its own non-nested SynIC
registers, restoring correct intercept/async-hypercall delivery. No
change for a non-nested (bare-metal) root.

Signed-off-by: Wei Hu <weh@microsoft.com>
---
 drivers/hv/mshv_synic.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/hv/mshv_synic.c b/drivers/hv/mshv_synic.c
index 7c168e5a740d..7d714ee5d226 100644
--- a/drivers/hv/mshv_synic.c
+++ b/drivers/hv/mshv_synic.c
@@ -472,7 +472,13 @@ static int mshv_synic_cpu_init(unsigned int cpu)
 	 * VMBus owns SIMP/SIEFP/SCONTROL when it is active.
 	 * See hv_hyp_synic_enable_regs() for that initialization.
 	 */
-	bool vmbus_active = hv_vmbus_exists();
+	/*
+	 * On a nested root partition VMBus programs the *nested* SynIC MSRs
+	 * (SIMP/SIEFP/SCONTROL) via hv_set_msr()'s nested remap, whereas the
+	 * code below reads/writes the non-nested SynIC MSRs. So when nested,
+	 * do not defer SynIC setup to VMBus -- set up our own registers.
+	 */
+	bool vmbus_active = hv_vmbus_exists() && !hv_nested;
 
 	/*
 	 * Map the SYNIC message page. When VMBus is not active the
@@ -593,7 +599,13 @@ static int mshv_synic_cpu_exit(unsigned int cpu)
 	struct hv_synic_event_ring_page **event_ring_page =
 		&spages->synic_event_ring_page;
 	/* VMBus owns SIMP/SIEFP/SCONTROL when it is active */
-	bool vmbus_active = hv_vmbus_exists();
+	/*
+	 * On a nested root partition VMBus programs the *nested* SynIC MSRs
+	 * (SIMP/SIEFP/SCONTROL) via hv_set_msr()'s nested remap, whereas the
+	 * code below reads/writes the non-nested SynIC MSRs. So when nested,
+	 * do not defer SynIC setup to VMBus -- set up our own registers.
+	 */
+	bool vmbus_active = hv_vmbus_exists() && !hv_nested;
 
 	/* Disable the interrupt */
 	sint.as_uint64 = hv_get_non_nested_msr(HV_MSR_SINT0 + HV_SYNIC_INTERCEPTION_SINT_INDEX);
-- 
2.43.0