kvm/x86: Hyper-V SynIC message slot pending clearing at SINT ack

The SynIC message protocol mandates that the message slot is claimed
by atomically setting message type to something other than HVMSG_NONE.
If another message is to be delivered while the slot is still busy,
message pending flag is asserted to indicate to the guest that the
hypervisor wants to be notified when the slot is released.

To make sure the protocol works regardless of where the message
sources are (kernel or userspace), clear the pending flag on SINT ACK
notification, and let the message sources compete for the slot again.

Signed-off-by: Andrey Smetanin <asmetanin@virtuozzo.com>
Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
CC: Gleb Natapov <gleb@kernel.org>
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: "K. Y. Srinivasan" <kys@microsoft.com>
CC: Haiyang Zhang <haiyangz@microsoft.com>
CC: Vitaly Kuznetsov <vkuznets@redhat.com>
CC: Roman Kagan <rkagan@virtuozzo.com>
CC: Denis V. Lunev <den@openvz.org>
CC: qemu-devel@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/hyperv.c

@@ -27,6 +27,7 @@
 #include "hyperv.h"
 
 #include <linux/kvm_host.h>
+#include <linux/highmem.h>
 #include <asm/apicdef.h>
 #include <trace/events/kvm.h>
 
@@ -116,13 +117,43 @@ static struct kvm_vcpu_hv_synic *synic_get(struct kvm *kvm, u32 vcpu_id)
 	return (synic->active) ? synic : NULL;
 }
 
+static void synic_clear_sint_msg_pending(struct kvm_vcpu_hv_synic *synic,
+					u32 sint)
+{
+	struct kvm_vcpu *vcpu = synic_to_vcpu(synic);
+	struct page *page;
+	gpa_t gpa;
+	struct hv_message *msg;
+	struct hv_message_page *msg_page;
+
+	gpa = synic->msg_page & PAGE_MASK;
+	page = kvm_vcpu_gfn_to_page(vcpu, gpa >> PAGE_SHIFT);
+	if (is_error_page(page)) {
+		vcpu_err(vcpu, "Hyper-V SynIC can't get msg page, gpa 0x%llx\n",
+			 gpa);
+		return;
+	}
+	msg_page = kmap_atomic(page);
+
+	msg = &msg_page->sint_message[sint];
+	msg->header.message_flags.msg_pending = 0;
+
+	kunmap_atomic(msg_page);
+	kvm_release_page_dirty(page);
+	kvm_vcpu_mark_page_dirty(vcpu, gpa >> PAGE_SHIFT);
+}
+
 static void kvm_hv_notify_acked_sint(struct kvm_vcpu *vcpu, u32 sint)
 {
 	struct kvm *kvm = vcpu->kvm;
+	struct kvm_vcpu_hv_synic *synic = vcpu_to_synic(vcpu);
 	int gsi, idx;
 
 	vcpu_debug(vcpu, "Hyper-V SynIC acked sint %d\n", sint);
 
+	if (synic->msg_page & HV_SYNIC_SIMP_ENABLE)
+		synic_clear_sint_msg_pending(synic, sint);
+
 	idx = srcu_read_lock(&kvm->irq_srcu);
 	gsi = atomic_read(&vcpu_to_synic(vcpu)->sint_to_gsi[sint]);
 	if (gsi != -1)

include/linux/kvm_host.h

@@ -450,6 +450,8 @@ struct kvm {
 
 #define vcpu_debug(vcpu, fmt, ...)					\
 	kvm_debug("vcpu%i " fmt, (vcpu)->vcpu_id, ## __VA_ARGS__)
+#define vcpu_err(vcpu, fmt, ...)					\
+	kvm_err("vcpu%i " fmt, (vcpu)->vcpu_id, ## __VA_ARGS__)
 
 static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
 {