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Subject[PATCH 5.16 183/186] x86/kvmclock: Fix Hyper-V Isolated VMs boot issue when vCPUs > 64
From: Dexuan Cui <>

commit 92e68cc558774de01024c18e8b35cdce4731c910 upstream.

When Linux runs as an Isolated VM on Hyper-V, it supports AMD SEV-SNP
but it's partially enlightened, i.e. cc_platform_has(
CC_ATTR_GUEST_MEM_ENCRYPT) is true but sev_active() is false.

Commit 4d96f9109109 per se is good, but with it now
kvm_setup_vsyscall_timeinfo() -> kvmclock_init_mem() calls
set_memory_decrypted(), and later gets stuck when trying to zere out
the pages pointed by 'hvclock_mem', if Linux runs as an Isolated VM on
Hyper-V. The cause is that here now the Linux VM should no longer access
the original guest physical addrss (GPA); instead the VM should do
memremap() and access the original GPA + ms_hyperv.shared_gpa_boundary:
see the example code in drivers/hv/connection.c: vmbus_connect() or
drivers/hv/ring_buffer.c: hv_ringbuffer_init(). If the VM tries to
access the original GPA, it keepts getting injected a fault by Hyper-V
and gets stuck there.

Here the issue happens only when the VM has >=65 vCPUs, because the
global static array hv_clock_boot[] can hold 64 "struct
pvclock_vsyscall_time_info" (the sizeof of the struct is 64 bytes), so
kvmclock_init_mem() only allocates memory in the case of vCPUs > 64.

Since the 'hvclock_mem' pages are only useful when the kvm clock is
supported by the underlying hypervisor, fix the issue by returning
early when Linux VM runs on Hyper-V, which doesn't support kvm clock.

Fixes: 4d96f9109109 ("x86/sev: Replace occurrences of sev_active() with cc_platform_has()")
Tested-by: Andrea Parri (Microsoft) <>
Signed-off-by: Andrea Parri (Microsoft) <>
Signed-off-by: Dexuan Cui <>
Message-Id: <>
Signed-off-by: Paolo Bonzini <>
Signed-off-by: Greg Kroah-Hartman <>
arch/x86/kernel/kvmclock.c | 3 +++
1 file changed, 3 insertions(+)

--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -239,6 +239,9 @@ static void __init kvmclock_init_mem(voi

static int __init kvm_setup_vsyscall_timeinfo(void)
+ if (!kvm_para_available())
+ return 0;

#ifdef CONFIG_X86_64

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