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KVM: x86/xen: Add event channel interrupt vector upcall

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It turns out that we can't handle event channels *entirely* in userspace
by delivering them as ExtINT, because KVM is a bit picky about when it
accepts ExtINT interrupts from a legacy PIC. The in-kernel local APIC
has to have LVT0 configured in APIC_MODE_EXTINT and unmasked, which
isn't necessarily the case for Xen guests especially on secondary CPUs.

To cope with this, add kvm_xen_get_interrupt() which checks the
evtchn_pending_upcall field in the Xen vcpu_info, and delivers the Xen
upcall vector (configured by KVM_XEN_ATTR_TYPE_UPCALL_VECTOR) if it's
set regardless of LAPIC LVT0 configuration. This gives us the minimum
support we need for completely userspace-based implementation of event
channels.

This does mean that vcpu_enter_guest() needs to check for the
evtchn_pending_upcall flag being set, because it can't rely on someone
having set KVM_REQ_EVENT unless we were to add some way for userspace to
do so manually.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
This commit is contained in:
David Woodhouse
2020-12-09 20:08:30 +00:00
parent f2340cd9e4
commit 40da8ccd72
6 changed files with 74 additions and 1 deletions
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2
include/uapi/linux/kvm.h
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@@ -1596,6 +1596,7 @@ struct kvm_xen_hvm_attr {
__u16 pad[3];
union {
__u8 long_mode;
__u8 vector;
struct {
__u64 gfn;
} shared_info;
@@ -1605,6 +1606,7 @@ struct kvm_xen_hvm_attr {
#define KVM_XEN_ATTR_TYPE_LONG_MODE 0x0
#define KVM_XEN_ATTR_TYPE_SHARED_INFO 0x1
#define KVM_XEN_ATTR_TYPE_UPCALL_VECTOR 0x2
/* Per-vCPU Xen attributes */
#define KVM_XEN_VCPU_GET_ATTR _IOWR(KVMIO, 0xca, struct kvm_xen_vcpu_attr)