[PATCH] hyperv: root partition faults writing to VP ASSIST MSR PAGE

[Praveen Kumar <kumarpraveen@linux.microsoft.com>]

The root partition is not supposed to write to VP ASSIST PAGE as this MSR
is specific to Guest VP, and thus below stack is observed.

[    2.778197] unchecked MSR access error: WRMSR to 0x40000073 (tried to write 0x0000000145ac5001) at rIP: 0xffffffff810c1084 (native_write_msr+0x4/0x30)
[    2.784867] Call Trace:
[    2.791507]  hv_cpu_init+0xf1/0x1c0
[    2.798144]  ? hyperv_report_panic+0xd0/0xd0
[    2.804806]  cpuhp_invoke_callback+0x11a/0x440
[    2.811465]  ? hv_resume+0x90/0x90
[    2.818137]  cpuhp_issue_call+0x126/0x130
[    2.824782]  __cpuhp_setup_state_cpuslocked+0x102/0x2b0
[    2.831427]  ? hyperv_report_panic+0xd0/0xd0
[    2.838075]  ? hyperv_report_panic+0xd0/0xd0
[    2.844723]  ? hv_resume+0x90/0x90
[    2.851375]  __cpuhp_setup_state+0x3d/0x90
[    2.858030]  hyperv_init+0x14e/0x410
[    2.864689]  ? enable_IR_x2apic+0x190/0x1a0
[    2.871349]  apic_intr_mode_init+0x8b/0x100
[    2.878017]  x86_late_time_init+0x20/0x30
[    2.884675]  start_kernel+0x459/0x4fb
[    2.891329]  secondary_startup_64_no_verify+0xb0/0xbb

Root partition actually shares the VP ASSIST page with hypervisor, and
thus as a solution, this patch memremaps the memory from hypervisor
during hv_cpu_init and unmaps during hv_cpu_die calls.

Further, this patch also resolve some error handling and checkpatch
errors

Signed-off-by: Praveen Kumar <kumarpraveen@linux.microsoft.com>
---
 arch/x86/hyperv/hv_init.c | 57 +++++++++++++++++++++++++++------------
 1 file changed, 40 insertions(+), 17 deletions(-)

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 6f247e7e07eb..292b17e0b173 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -44,7 +44,7 @@ EXPORT_SYMBOL_GPL(hv_vp_assist_page);
 
 static int hv_cpu_init(unsigned int cpu)
 {
-	struct hv_vp_assist_page **hvp = &hv_vp_assist_page[smp_processor_id()];
+	struct hv_vp_assist_page **hvp = NULL;
 	int ret;
 
 	ret = hv_common_cpu_init(cpu);
@@ -54,25 +54,43 @@ static int hv_cpu_init(unsigned int cpu)
 	if (!hv_vp_assist_page)
 		return 0;
 
+	hvp = &hv_vp_assist_page[smp_processor_id()];
+
 	/*
-	 * The VP ASSIST PAGE is an "overlay" page (see Hyper-V TLFS's Section
-	 * 5.2.1 "GPA Overlay Pages"). Here it must be zeroed out to make sure
-	 * we always write the EOI MSR in hv_apic_eoi_write() *after* the
-	 * EOI optimization is disabled in hv_cpu_die(), otherwise a CPU may
-	 * not be stopped in the case of CPU offlining and the VM will hang.
+	 * For Root partition we need to map the hypervisor VP ASSIST PAGE
+	 * instead of allocating a new page.
 	 */
-	if (!*hvp) {
-		*hvp = __vmalloc(PAGE_SIZE, GFP_KERNEL | __GFP_ZERO);
-	}
+	if (hv_root_partition &&
+	    ms_hyperv.features & HV_MSR_APIC_ACCESS_AVAILABLE) {
+		union hv_x64_msr_hypercall_contents hypercall_msr;
+
+		rdmsrl(HV_X64_MSR_VP_ASSIST_PAGE, hypercall_msr.as_uint64);
+		/* remapping to root partition address space */
+		*hvp = memremap(hypercall_msr.guest_physical_address <<
+					HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT,
+				PAGE_SIZE, MEMREMAP_WB);
+		WARN_ON(!(*hvp));
+	} else {
+		/*
+		 * The VP ASSIST PAGE is an "overlay" page (see Hyper-V TLFS's
+		 * Section 5.2.1 "GPA Overlay Pages"). Here it must be zeroed
+		 * out to make sure we always write the EOI MSR in
+		 * hv_apic_eoi_write() *after* the EOI optimization is disabled
+		 * in hv_cpu_die(), otherwise a CPU may not be stopped in the
+		 * case of CPU offlining and the VM will hang.
+		 */
+		if (!*hvp)
+			*hvp = __vmalloc(PAGE_SIZE, GFP_KERNEL | __GFP_ZERO);
 
-	if (*hvp) {
-		u64 val;
+		if (*hvp) {
+			u64 val;
 
-		val = vmalloc_to_pfn(*hvp);
-		val = (val << HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT) |
-			HV_X64_MSR_VP_ASSIST_PAGE_ENABLE;
+			val = vmalloc_to_pfn(*hvp);
+			val = (val << HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT) |
+				HV_X64_MSR_VP_ASSIST_PAGE_ENABLE;
 
-		wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, val);
+			wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, val);
+		}
 	}
 
 	return 0;
@@ -170,8 +188,13 @@ static int hv_cpu_die(unsigned int cpu)
 
 	hv_common_cpu_die(cpu);
 
-	if (hv_vp_assist_page && hv_vp_assist_page[cpu])
-		wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, 0);
+	if (hv_vp_assist_page && hv_vp_assist_page[cpu]) {
+		if (hv_root_partition &&
+		    ms_hyperv.features & HV_MSR_APIC_ACCESS_AVAILABLE)
+			memunmap(hv_vp_assist_page[cpu]);
+		else
+			wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, 0);
+	}
 
 	if (hv_reenlightenment_cb == NULL)
 		return 0;
-- 
2.23.4