KVM: PPC: Book3S HV: XIVE: Make VP block size configurable

The XIVE VP is an internal structure which allow the XIVE interrupt
controller to maintain the interrupt context state of vCPUs non
dispatched on HW threads.

When a guest is started, the XIVE KVM device allocates a block of
XIVE VPs in OPAL, enough to accommodate the highest possible vCPU
id KVM_MAX_VCPU_ID (16384) packed down to KVM_MAX_VCPUS (2048).
With a guest's core stride of 8 and a threading mode of 1 (QEMU's
default), a VM must run at least 256 vCPUs to actually need such a
range of VPs.

A POWER9 system has a limited XIVE VP space : 512k and KVM is
currently wasting this HW resource with large VP allocations,
especially since a typical VM likely runs with a lot less vCPUs.

Make the size of the VP block configurable. Add an nr_servers
field to the XIVE structure and a function to set it for this
purpose.

Split VP allocation out of the device create function. Since the
VP block isn't used before the first vCPU connects to the XIVE KVM
device, allocation is now performed by kvmppc_xive_connect_vcpu().
This gives the opportunity to set nr_servers in between:

          kvmppc_xive_create() / kvmppc_xive_native_create()
                               .
                               .
                     kvmppc_xive_set_nr_servers()
                               .
                               .
    kvmppc_xive_connect_vcpu() / kvmppc_xive_native_connect_vcpu()

The connect_vcpu() functions check that the vCPU id is below nr_servers
and if it is the first vCPU they allocate the VP block. This is protected
against a concurrent update of nr_servers by kvmppc_xive_set_nr_servers()
with the xive->lock mutex.

Also, the block is allocated once for the device lifetime: nr_servers
should stay constant otherwise connect_vcpu() could generate a boggus
VP id and likely crash OPAL. It is thus forbidden to update nr_servers
once the block is allocated.

If the VP allocation fail, return ENOSPC which seems more appropriate to
report the depletion of system wide HW resource than ENOMEM or ENXIO.

A VM using a stride of 8 and 1 thread per core with 32 vCPUs would hence
only need 256 VPs instead of 2048. If the stride is set to match the number
of threads per core, this goes further down to 32.

This will be exposed to userspace by a subsequent patch.
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

arch/powerpc/kvm/book3s_xive.c

@@ -1213,13 +1213,13 @@ void kvmppc_xive_cleanup_vcpu(struct kvm_vcpu *vcpu)
 
 static bool kvmppc_xive_vcpu_id_valid(struct kvmppc_xive *xive, u32 cpu)
 {
-	/* We have a block of KVM_MAX_VCPUS VPs. We just need to check
+	/* We have a block of xive->nr_servers VPs. We just need to check
 	 * raw vCPU ids are below the expected limit for this guest's
 	 * core stride ; kvmppc_pack_vcpu_id() will pack them down to an
 	 * index that can be safely used to compute a VP id that belongs
 	 * to the VP block.
 	 */
-	return cpu < KVM_MAX_VCPUS * xive->kvm->arch.emul_smt_mode;
+	return cpu < xive->nr_servers * xive->kvm->arch.emul_smt_mode;
 }
 
 int kvmppc_xive_compute_vp_id(struct kvmppc_xive *xive, u32 cpu, u32 *vp)
@@ -1231,6 +1231,14 @@ int kvmppc_xive_compute_vp_id(struct kvmppc_xive *xive, u32 cpu, u32 *vp)
 		return -EINVAL;
 	}
 
+	if (xive->vp_base == XIVE_INVALID_VP) {
+		xive->vp_base = xive_native_alloc_vp_block(xive->nr_servers);
+		pr_devel("VP_Base=%x nr_servers=%d\n", xive->vp_base, xive->nr_servers);
+
+		if (xive->vp_base == XIVE_INVALID_VP)
+			return -ENOSPC;
+	}
+
 	vp_id = kvmppc_xive_vp(xive, cpu);
 	if (kvmppc_xive_vp_in_use(xive->kvm, vp_id)) {
 		pr_devel("Duplicate !\n");
@@ -1858,6 +1866,43 @@ int kvmppc_xive_set_irq(struct kvm *kvm, int irq_source_id, u32 irq, int level,
 	return 0;
 }
 
+int kvmppc_xive_set_nr_servers(struct kvmppc_xive *xive, u64 addr)
+{
+	u32 __user *ubufp = (u32 __user *) addr;
+	u32 nr_servers;
+	int rc = 0;
+
+	if (get_user(nr_servers, ubufp))
+		return -EFAULT;
+
+	pr_devel("%s nr_servers=%u\n", __func__, nr_servers);
+
+	if (!nr_servers || nr_servers > KVM_MAX_VCPU_ID)
+		return -EINVAL;
+
+	mutex_lock(&xive->lock);
+	if (xive->vp_base != XIVE_INVALID_VP)
+		/* The VP block is allocated once and freed when the device
+		 * is released. Better not allow to change its size since its
+		 * used by connect_vcpu to validate vCPU ids are valid (eg,
+		 * setting it back to a higher value could allow connect_vcpu
+		 * to come up with a VP id that goes beyond the VP block, which
+		 * is likely to cause a crash in OPAL).
+		 */
+		rc = -EBUSY;
+	else if (nr_servers > KVM_MAX_VCPUS)
+		/* We don't need more servers. Higher vCPU ids get packed
+		 * down below KVM_MAX_VCPUS by kvmppc_pack_vcpu_id().
+		 */
+		xive->nr_servers = KVM_MAX_VCPUS;
+	else
+		xive->nr_servers = nr_servers;
+
+	mutex_unlock(&xive->lock);
+
+	return rc;
+}
+
 static int xive_set_attr(struct kvm_device *dev, struct kvm_device_attr *attr)
 {
 	struct kvmppc_xive *xive = dev->private;
@@ -2025,7 +2070,6 @@ static int kvmppc_xive_create(struct kvm_device *dev, u32 type)
 {
 	struct kvmppc_xive *xive;
 	struct kvm *kvm = dev->kvm;
-	int ret = 0;
 
 	pr_devel("Creating xive for partition\n");
 
@@ -2049,18 +2093,15 @@ static int kvmppc_xive_create(struct kvm_device *dev, u32 type)
 	else
 		xive->q_page_order = xive->q_order - PAGE_SHIFT;
 
-	/* Allocate a bunch of VPs */
-	xive->vp_base = xive_native_alloc_vp_block(KVM_MAX_VCPUS);
-	pr_devel("VP_Base=%x\n", xive->vp_base);
-
-	if (xive->vp_base == XIVE_INVALID_VP)
-		ret = -ENOMEM;
+	/* VP allocation is delayed to the first call to connect_vcpu */
+	xive->vp_base = XIVE_INVALID_VP;
+	/* KVM_MAX_VCPUS limits the number of VMs to roughly 64 per sockets
+	 * on a POWER9 system.
+	 */
+	xive->nr_servers = KVM_MAX_VCPUS;
 
 	xive->single_escalation = xive_native_has_single_escalation();
 
-	if (ret)
-		return ret;
-
 	kvm->arch.xive = xive;
 	return 0;
 }

arch/powerpc/kvm/book3s_xive.h

@@ -135,6 +135,9 @@ struct kvmppc_xive {
 	/* Flags */
 	u8	single_escalation;
 
+	/* Number of entries in the VP block */
+	u32	nr_servers;
+
 	struct kvmppc_xive_ops *ops;
 	struct address_space   *mapping;
 	struct mutex mapping_lock;
@@ -297,6 +300,7 @@ struct kvmppc_xive *kvmppc_xive_get_device(struct kvm *kvm, u32 type);
 void xive_cleanup_single_escalation(struct kvm_vcpu *vcpu,
 				    struct kvmppc_xive_vcpu *xc, int irq);
 int kvmppc_xive_compute_vp_id(struct kvmppc_xive *xive, u32 cpu, u32 *vp);
+int kvmppc_xive_set_nr_servers(struct kvmppc_xive *xive, u64 addr);
 
 #endif /* CONFIG_KVM_XICS */
 #endif /* _KVM_PPC_BOOK3S_XICS_H */

arch/powerpc/kvm/book3s_xive_native.c

@@ -1060,7 +1060,6 @@ static int kvmppc_xive_native_create(struct kvm_device *dev, u32 type)
 {
 	struct kvmppc_xive *xive;
 	struct kvm *kvm = dev->kvm;
-	int ret = 0;
 
 	pr_devel("Creating xive native device\n");
 
@@ -1077,23 +1076,16 @@ static int kvmppc_xive_native_create(struct kvm_device *dev, u32 type)
 	mutex_init(&xive->mapping_lock);
 	mutex_init(&xive->lock);
 
-	/*
-	 * Allocate a bunch of VPs. KVM_MAX_VCPUS is a large value for
-	 * a default. Getting the max number of CPUs the VM was
-	 * configured with would improve our usage of the XIVE VP space.
+	/* VP allocation is delayed to the first call to connect_vcpu */
+	xive->vp_base = XIVE_INVALID_VP;
+	/* KVM_MAX_VCPUS limits the number of VMs to roughly 64 per sockets
+	 * on a POWER9 system.
 	 */
-	xive->vp_base = xive_native_alloc_vp_block(KVM_MAX_VCPUS);
-	pr_devel("VP_Base=%x\n", xive->vp_base);
-
-	if (xive->vp_base == XIVE_INVALID_VP)
-		ret = -ENXIO;
+	xive->nr_servers = KVM_MAX_VCPUS;
 
 	xive->single_escalation = xive_native_has_single_escalation();
 	xive->ops = &kvmppc_xive_native_ops;
 
-	if (ret)
-		return ret;
-
 	kvm->arch.xive = xive;
 	return 0;
 }