[PATCH 2/4] KVM: x86: Document APIC base address constraint for in-kernel irqchip

[Tim Wiederhake]

When an in-kernel irqchip is enabled, vcpu creation installs a private
4 KiB memory slot at the default APIC base address (0xfee00000).  If a
user memory region overlaps this slot, vcpu creation fails with EEXIST.
The same error occurs when installing an overlapping user memory region
after vcpu creation.

This constraint is not documented anywhere.  Add a note to the
KVM_CREATE_IRQCHIP documentation.

Signed-off-by: Tim Wiederhake <twiederh@xxxxxxxxxx>
---
 Documentation/virt/kvm/api.rst | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index ec5bf99ff8b8..da8beb80699a 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -863,6 +863,11 @@ KVM_CREATE_DEVICE, which also supports creating a GICv2.  Using
 KVM_CREATE_DEVICE is preferred over KVM_CREATE_IRQCHIP for GICv2.
 On s390, a dummy irq routing table is created.
 
+On x86, when an in-kernel irqchip is enabled, KVM reserves a private memory
+slot for the local APIC at the default APIC base address (0xfee00000).  User
+space must not map guest memory that covers this address and must leave a 4 KiB
+hole in the guest physical memory map at this address.
+
 Note that on s390 the KVM_CAP_S390_IRQCHIP vm capability needs to be enabled
 before KVM_CREATE_IRQCHIP can be used.
 
-- 
2.52.0