powerpc/vas: Add paste address mmap fault handler

The user space opens VAS windows and issues NX requests by pasting
CRB on the corresponding paste address mmap. When the system lost
credits due to core removal, the kernel has to close the window in
the hypervisor and make the window inactive by unmapping this paste
address. Also the OS has to handle NX request page faults if the user
space issue NX requests.

This handler maps the new paste address with the same VMA when the
window is active again (due to core add with DLPAR). Otherwise
returns paste failure.
Signed-off-by: Haren Myneni <haren@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/3956e1c1fdfde69127055ff1c0256c7d71104030.camel@linux.ibm.com

arch/powerpc/include/asm/vas.h

@@ -29,6 +29,12 @@
 #define VAS_THRESH_FIFO_GT_QTR_FULL	2
 #define VAS_THRESH_FIFO_GT_EIGHTH_FULL	3
 
+/*
+ * VAS window Linux status bits
+ */
+#define VAS_WIN_ACTIVE		0x0	/* Used in platform independent */
+					/* vas mmap() */
+
 /*
  * Get/Set bit fields
  */
@@ -59,6 +65,9 @@ struct vas_user_win_ref {
 	struct pid *pid;	/* PID of owner */
 	struct pid *tgid;	/* Thread group ID of owner */
 	struct mm_struct *mm;	/* Linux process mm_struct */
+	struct mutex mmap_mutex;	/* protects paste address mmap() */
+					/* with DLPAR close/open windows */
+	struct vm_area_struct *vma;	/* Save VMA and used in DLPAR ops */
 };
 
 /*
@@ -67,6 +76,7 @@ struct vas_user_win_ref {
 struct vas_window {
 	u32 winid;
 	u32 wcreds_max;	/* Window credits */
+	u32 status;	/* Window status used in OS */
 	enum vas_cop_type cop;
 	struct vas_user_win_ref task_ref;
 	char *dbgname;

arch/powerpc/platforms/book3s/vas-api.c

@@ -316,6 +316,7 @@ static int coproc_ioc_tx_win_open(struct file *fp, unsigned long arg)
 		return PTR_ERR(txwin);
 	}
 
+	mutex_init(&txwin->task_ref.mmap_mutex);
 	cp_inst->txwin = txwin;
 
 	return 0;
@@ -350,6 +351,70 @@ static int coproc_release(struct inode *inode, struct file *fp)
 	return 0;
 }
 
+/*
+ * This fault handler is invoked when the core generates page fault on
+ * the paste address. Happens if the kernel closes window in hypervisor
+ * (on pseries) due to lost credit or the paste address is not mapped.
+ */
+static vm_fault_t vas_mmap_fault(struct vm_fault *vmf)
+{
+	struct vm_area_struct *vma = vmf->vma;
+	struct file *fp = vma->vm_file;
+	struct coproc_instance *cp_inst = fp->private_data;
+	struct vas_window *txwin;
+	vm_fault_t fault;
+	u64 paste_addr;
+
+	/*
+	 * window is not opened. Shouldn't expect this error.
+	 */
+	if (!cp_inst || !cp_inst->txwin) {
+		pr_err("%s(): Unexpected fault on paste address with TX window closed\n",
+				__func__);
+		return VM_FAULT_SIGBUS;
+	}
+
+	txwin = cp_inst->txwin;
+	/*
+	 * When the LPAR lost credits due to core removal or during
+	 * migration, invalidate the existing mapping for the current
+	 * paste addresses and set windows in-active (zap_page_range in
+	 * reconfig_close_windows()).
+	 * New mapping will be done later after migration or new credits
+	 * available. So continue to receive faults if the user space
+	 * issue NX request.
+	 */
+	if (txwin->task_ref.vma != vmf->vma) {
+		pr_err("%s(): No previous mapping with paste address\n",
+			__func__);
+		return VM_FAULT_SIGBUS;
+	}
+
+	mutex_lock(&txwin->task_ref.mmap_mutex);
+	/*
+	 * The window may be inactive due to lost credit (Ex: core
+	 * removal with DLPAR). If the window is active again when
+	 * the credit is available, map the new paste address at the
+	 * the window virtual address.
+	 */
+	if (txwin->status == VAS_WIN_ACTIVE) {
+		paste_addr = cp_inst->coproc->vops->paste_addr(txwin);
+		if (paste_addr) {
+			fault = vmf_insert_pfn(vma, vma->vm_start,
+					(paste_addr >> PAGE_SHIFT));
+			mutex_unlock(&txwin->task_ref.mmap_mutex);
+			return fault;
+		}
+	}
+	mutex_unlock(&txwin->task_ref.mmap_mutex);
+
+	return VM_FAULT_SIGBUS;
+}
+
+static const struct vm_operations_struct vas_vm_ops = {
+	.fault = vas_mmap_fault,
+};
+
 static int coproc_mmap(struct file *fp, struct vm_area_struct *vma)
 {
 	struct coproc_instance *cp_inst = fp->private_data;
@@ -398,6 +463,9 @@ static int coproc_mmap(struct file *fp, struct vm_area_struct *vma)
 	pr_devel("%s(): paste addr %llx at %lx, rc %d\n", __func__,
 			paste_addr, vma->vm_start, rc);
 
+	txwin->task_ref.vma = vma;
+	vma->vm_ops = &vas_vm_ops;
+
 	return rc;
 }