cxl: Add alternate MMIO error handling

userspace programs using cxl currently have to use two strategies for
dealing with MMIO errors simultaneously. They have to check every read
for a return of all Fs in case the adapter has gone away and the kernel
has not yet noticed, and they have to deal with SIGBUS in case the
kernel has already noticed, invalidated the mapping and marked the
context as failed.

In order to simplify things, this patch adds an alternative approach
where the kernel will return a page filled with Fs instead of delivering
a SIGBUS. This allows userspace to only need to deal with one of these
two error paths, and is intended for use in libraries that use cxl
transparently and may not be able to safely install a signal handler.

This approach will only work if certain constraints are met. Namely, if
the application is both reading and writing to an address in the problem
state area it cannot assume that a non-FF read is OK, as it may just be
reading out a value it has previously written. Further - since only one
page is used per context a write to a given offset would be visible when
reading the same offset from a different page in the mapping (this only
applies within a single context, not between contexts).

An application could deal with this by e.g. making sure it also reads
from a read-only offset after any reads to a read/write offset.

Due to these constraints, this functionality must be explicitly
requested by userspace when starting the context by passing in the
CXL_START_WORK_ERR_FF flag.

Signed-off-by: Ian Munsie <[e-mail]>
Acked-by: Michael Neuling <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>

drivers/misc/cxl/context.c

···
 	if (ctx->status != STARTED) {
 		mutex_unlock(&ctx->status_mutex);
 		pr_devel("%s: Context not started, failing problem state access\n", __func__);
+		if (ctx->mmio_err_ff) {
+			if (!ctx->ff_page) {
+				ctx->ff_page = alloc_page(GFP_USER);
+				if (!ctx->ff_page)
+					return VM_FAULT_OOM;
+				memset(page_address(ctx->ff_page), 0xff, PAGE_SIZE);
+			}
+			get_page(ctx->ff_page);
+			vmf->page = ctx->ff_page;
+			vma->vm_page_prot = pgprot_cached(vma->vm_page_prot);
+			return 0;
+		}
 		return VM_FAULT_SIGBUS;
 	}
 
···
 	struct cxl_context *ctx = container_of(rcu, struct cxl_context, rcu);
 
 	free_page((u64)ctx->sstp);
+	if (ctx->ff_page)
+		__free_page(ctx->ff_page);
 	ctx->sstp = NULL;
 
 	kfree(ctx);

drivers/misc/cxl/cxl.h

···
  * Bump version each time a user API change is made, whether it is
  * backwards compatible ot not.
  */
-#define CXL_API_VERSION 1
+#define CXL_API_VERSION 2
 #define CXL_API_VERSION_COMPATIBLE 1
 
 /*
···
 	/* Used to unmap any mmaps when force detaching */
 	struct address_space *mapping;
 	struct mutex mapping_lock;
+	struct page *ff_page;
+	bool mmio_err_ff;
 
 	spinlock_t sste_lock; /* Protects segment table entries */
 	struct cxl_sste *sstp;

drivers/misc/cxl/file.c

···
 	if (work.flags & CXL_START_WORK_AMR)
 		amr = work.amr & mfspr(SPRN_UAMOR);
 
+	ctx->mmio_err_ff = !!(work.flags & CXL_START_WORK_ERR_FF);
+
 	/*
 	 * We grab the PID here and not in the file open to allow for the case
 	 * where a process (master, some daemon, etc) has opened the chardev on
···
 	 * If these change we really need to update API.  Either change some
 	 * flags or update API version number CXL_API_VERSION.
 	 */
-	BUILD_BUG_ON(CXL_API_VERSION != 1);
+	BUILD_BUG_ON(CXL_API_VERSION != 2);
 	BUILD_BUG_ON(sizeof(struct cxl_ioctl_start_work) != 64);
 	BUILD_BUG_ON(sizeof(struct cxl_event_header) != 8);
 	BUILD_BUG_ON(sizeof(struct cxl_event_afu_interrupt) != 8);

include/uapi/misc/cxl.h

···
 
 #define CXL_START_WORK_AMR		0x0000000000000001ULL
 #define CXL_START_WORK_NUM_IRQS		0x0000000000000002ULL
+#define CXL_START_WORK_ERR_FF		0x0000000000000004ULL
 #define CXL_START_WORK_ALL		(CXL_START_WORK_AMR |\
-					 CXL_START_WORK_NUM_IRQS)
+					 CXL_START_WORK_NUM_IRQS |\
+					 CXL_START_WORK_ERR_FF)
 
 
 /* Possible modes that an afu can be in */