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From:   Praveen Kumar <kumarpraveen@linux.microsoft.com>
To:     linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     kys@microsoft.com, haiyangz@microsoft.com, sthemmin@microsoft.com,
        wei.liu@kernel.org, decui@microsoft.com, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com,
        viremana@linux.microsoft.com, sunilmut@microsoft.com,
        nunodasneves@linux.microsoft.com
Subject: [PATCH v2] hyperv: root partition faults writing to VP ASSIST MSR PAGE
Date:   Wed, 21 Jul 2021 23:33:02 +0530
Message-Id: <20210721180302.18764-1-kumarpraveen@linux.microsoft.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

For Root partition the VP assist pages are pre-determined by the
hypervisor. The Root kernel is not allowed to change them to
different locations. And thus, we are getting below stack as in
current implementation Root is trying to perform write to specific
MSR.

[ 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

Since, the hypervisor already provides the VP assist page for root
partition, we need to memremaps the memory from hypervisor for root
kernel to use. The mapping is done in hv_cpu_init during bringup and
is unmaped in hv_cpu_die during teardown.

Signed-off-by: Praveen Kumar <kumarpraveen@linux.microsoft.com>
---
 arch/x86/hyperv/hv_init.c | 53 ++++++++++++++++++++++++++-------------
 1 file changed, 36 insertions(+), 17 deletions(-)

changelog:
v1: initial patch
v2: commit message changes, removal of HV_MSR_APIC_ACCESS_AVAILABLE
    check and addition of null check before reading the VP assist MSR
    for root partition

---
diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 6f247e7e07eb..ffd3d3b37235 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -55,26 +55,41 @@ static int hv_cpu_init(unsigned int cpu)
 		return 0;
 
 	/*
-	 * 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) {
+		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 */
+		if (!*hvp)
+			*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* theEOI 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 +185,12 @@ 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)
+			memunmap(hv_vp_assist_page[cpu]);
+		else
+			wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, 0);
+	}
 
 	if (hv_reenlightenment_cb == NULL)
 		return 0;
-- 
2.25.1