Patch "x86/hyperv: Move required MSRs check to initial platform probing" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    x86/hyperv: Move required MSRs check to initial platform probing

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     x86-hyperv-move-required-msrs-check-to-initial-platf.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit e6e9bf6b0aab873815181dfbd6e9f8748475147c
Author: Sean Christopherson <seanjc@xxxxxxxxxx>
Date:   Thu Nov 4 18:22:39 2021 +0000

    x86/hyperv: Move required MSRs check to initial platform probing
    
    [ Upstream commit f3e613e72f66226b3bea1046c1b864f67a3000a4 ]
    
    Explicitly check for MSR_HYPERCALL and MSR_VP_INDEX support when probing
    for running as a Hyper-V guest instead of waiting until hyperv_init() to
    detect the bogus configuration.  Add messages to give the admin a heads
    up that they are likely running on a broken virtual machine setup.
    
    At best, silently disabling Hyper-V is confusing and difficult to debug,
    e.g. the kernel _says_ it's using all these fancy Hyper-V features, but
    always falls back to the native versions.  At worst, the half baked setup
    will crash/hang the kernel.
    
    Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
    Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
    Link: https://lore.kernel.org/r/20211104182239.1302956-3-seanjc@xxxxxxxxxx
    Signed-off-by: Wei Liu <wei.liu@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index d20eef3d452a5..b6d48ca5b0f17 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -348,20 +348,13 @@ static void __init hv_get_partition_id(void)
  */
 void __init hyperv_init(void)
 {
-	u64 guest_id, required_msrs;
+	u64 guest_id;
 	union hv_x64_msr_hypercall_contents hypercall_msr;
 	int cpuhp;
 
 	if (x86_hyper_type != X86_HYPER_MS_HYPERV)
 		return;
 
-	/* Absolutely required MSRs */
-	required_msrs = HV_MSR_HYPERCALL_AVAILABLE |
-		HV_MSR_VP_INDEX_AVAILABLE;
-
-	if ((ms_hyperv.features & required_msrs) != required_msrs)
-		return;
-
 	if (hv_common_init())
 		return;
 
diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index e095c28d27ae8..ef6316fef99ff 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -163,12 +163,22 @@ static uint32_t  __init ms_hyperv_platform(void)
 	cpuid(HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS,
 	      &eax, &hyp_signature[0], &hyp_signature[1], &hyp_signature[2]);
 
-	if (eax >= HYPERV_CPUID_MIN &&
-	    eax <= HYPERV_CPUID_MAX &&
-	    !memcmp("Microsoft Hv", hyp_signature, 12))
-		return HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS;
+	if (eax < HYPERV_CPUID_MIN || eax > HYPERV_CPUID_MAX ||
+	    memcmp("Microsoft Hv", hyp_signature, 12))
+		return 0;
 
-	return 0;
+	/* HYPERCALL and VP_INDEX MSRs are mandatory for all features. */
+	eax = cpuid_eax(HYPERV_CPUID_FEATURES);
+	if (!(eax & HV_MSR_HYPERCALL_AVAILABLE)) {
+		pr_warn("x86/hyperv: HYPERCALL MSR not available.\n");
+		return 0;
+	}
+	if (!(eax & HV_MSR_VP_INDEX_AVAILABLE)) {
+		pr_warn("x86/hyperv: VP_INDEX MSR not available.\n");
+		return 0;
+	}
+
+	return HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS;
 }
 
 static unsigned char hv_get_nmi_reason(void)