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Subject[XEN PATCH 1/2] hvm: Support more than 32 VCPUS when migrating.
From: Konrad Rzeszutek Wilk <>

When we migrate an HVM guest, by default our shared_info can
only hold up to 32 CPUs. As such the hypercall
VCPUOP_register_vcpu_info was introduced which allowed us to
setup per-page areas for VCPUs. This means we can boot PVHVM
guest with more than 32 VCPUs. During migration the per-cpu
structure is allocated fresh by the hypervisor (vcpu_info_mfn
is set to INVALID_MFN) so that the newly migrated guest
can do make the VCPUOP_register_vcpu_info hypercall.

Unfortunatly we end up triggering this condition:
/* Run this command on yourself or on other offline VCPUS. */
if ( (v != current) && !test_bit(_VPF_down, &v->pause_flags) )

which means we are unable to setup the per-cpu VCPU structures
for running vCPUS. The Linux PV code paths make this work by
iterating over every vCPU with:

1) is target CPU up (VCPUOP_is_up hypercall?)
2) if yes, then VCPUOP_down to pause it.
3) VCPUOP_register_vcpu_info
4) if it was down, then VCPUOP_up to bring it back up

But since VCPUOP_down, VCPUOP_is_up, and VCPUOP_up are
not allowed on HVM guests we can't do this. This patch
enables this.

Signed-off-by: Konrad Rzeszutek Wilk <>
xen/arch/x86/hvm/hvm.c | 3 +++
1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 38c491e..b5b92fe 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -3470,6 +3470,9 @@ static long hvm_vcpu_op(
case VCPUOP_stop_singleshot_timer:
case VCPUOP_register_vcpu_info:
case VCPUOP_register_vcpu_time_memory_area:
+ case VCPUOP_down:
+ case VCPUOP_up:
+ case VCPUOP_is_up:
rc = do_vcpu_op(cmd, vcpuid, arg);

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