Re: [PATCH v5 02/16] x86/hyperv: detect if Linux is the root partition

On Wed, Jan 20, 2021 at 11:03:18AM -0500, Pavel Tatashin wrote:

On Wed, Jan 20, 2021 at 7:01 AM Wei Liu [off-list ref] wrote:

quoted

For now we can use the privilege flag to check. Stash the value to be
used later.

Put in a bunch of defines for future use when we want to have more
fine-grained detection.

Signed-off-by: Wei Liu <wei.liu@kernel.org>
---
v3: move hv_root_partition to mshyperv.c
---
 arch/x86/include/asm/hyperv-tlfs.h | 10 ++++++++++
 arch/x86/include/asm/mshyperv.h    |  2 ++
 arch/x86/kernel/cpu/mshyperv.c     | 20 ++++++++++++++++++++
 3 files changed, 32 insertions(+)

diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-tlfs.h
index 6bf42aed387e..204010350604 100644
--- a/arch/x86/include/asm/hyperv-tlfs.h
+++ b/arch/x86/include/asm/hyperv-tlfs.h

@@ -21,6 +21,7 @@
 #define HYPERV_CPUID_FEATURES                  0x40000003
 #define HYPERV_CPUID_ENLIGHTMENT_INFO          0x40000004
 #define HYPERV_CPUID_IMPLEMENT_LIMITS          0x40000005
+#define HYPERV_CPUID_CPU_MANAGEMENT_FEATURES   0x40000007
 #define HYPERV_CPUID_NESTED_FEATURES           0x4000000A

 #define HYPERV_CPUID_VIRT_STACK_INTERFACE      0x40000081

@@ -110,6 +111,15 @@
 /* Recommend using enlightened VMCS */
 #define HV_X64_ENLIGHTENED_VMCS_RECOMMENDED            BIT(14)

+/*
+ * CPU management features identification.
+ * These are HYPERV_CPUID_CPU_MANAGEMENT_FEATURES.EAX bits.
+ */
+#define HV_X64_START_LOGICAL_PROCESSOR                 BIT(0)
+#define HV_X64_CREATE_ROOT_VIRTUAL_PROCESSOR           BIT(1)
+#define HV_X64_PERFORMANCE_COUNTER_SYNC                        BIT(2)
+#define HV_X64_RESERVED_IDENTITY_BIT                   BIT(31)
+
 /*
  * Virtual processor will never share a physical core with another virtual
  * processor, except for virtual processors that are reported as sibling SMT

diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
index ffc289992d1b..ac2b0d110f03 100644
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h

@@ -237,6 +237,8 @@ int hyperv_fill_flush_guest_mapping_list(
                struct hv_guest_mapping_flush_list *flush,
                u64 start_gfn, u64 end_gfn);

+extern bool hv_root_partition;
+
 #ifdef CONFIG_X86_64
 void hv_apic_init(void);
 void __init hv_init_spinlocks(void);

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index f628e3dc150f..c376d191a260 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c

@@ -32,6 +32,10 @@
 #include <asm/nmi.h>
 #include <clocksource/hyperv_timer.h>

+/* Is Linux running as the root partition? */
+bool hv_root_partition;
+EXPORT_SYMBOL_GPL(hv_root_partition);
+
 struct ms_hyperv_info ms_hyperv;
 EXPORT_SYMBOL_GPL(ms_hyperv);

@@ -237,6 +241,22 @@ static void __init ms_hyperv_init_platform(void)
        pr_debug("Hyper-V: max %u virtual processors, %u logical processors\n",
                 ms_hyperv.max_vp_index, ms_hyperv.max_lp_index);

+       /*
+        * Check CPU management privilege.
+        *
+        * To mirror what Windows does we should extract CPU management
+        * features and use the ReservedIdentityBit to detect if Linux is the
+        * root partition. But that requires negotiating CPU management
+        * interface (a process to be finalized).

Is this comment relevant? Do we have to mirror what Windows does?

We should do that in the future when the process for negotiating CPU
management features is stabilized / finalized.

quoted

+        *
+        * For now, use the privilege flag as the indicator for running as
+        * root.
+        */
+       if (cpuid_ebx(HYPERV_CPUID_FEATURES) & HV_CPU_MANAGEMENT) {
+               hv_root_partition = true;
+               pr_info("Hyper-V: running as root partition\n");
+       }
+

Reviewed-by: Pavel Tatashin <pasha.tatashin@soleen.com>

Thanks for reviewing these patches.

Wei.