s390/mm: rework memcpy_real() to avoid DAT-off mode

Function memcpy_real() is an univeral data mover that does not
require DAT mode to be able reading from a physical address.
Its advantage is an ability to read from any address, even
those for which no kernel virtual mapping exists.

Although memcpy_real() is interrupt-safe, there are no handlers
that make use of this function. The compiler instrumentation
have to be disabled and separate no-DAT stack used to allow
execution of the function once DAT mode is disabled.

Rework memcpy_real() to overcome these shortcomings. As result,
data copying (which is primarily reading out a crashed system
memory by a user process) is executed on a regular stack with
enabled interrupts. Also, use of memcpy_real_buf swap buffer
becomes unnecessary and the swapping is eliminated.

The above is achieved by using a fixed virtual address range
that spans a single page and remaps that page repeatedly when
memcpy_real() is called for a particular physical address.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/arch/s390/include/asm/maccess.h b/arch/s390/include/asm/maccess.h
new file mode 100644
index 000000000000..21ebd9bf34cc
--- /dev/null
+++ b/arch/s390/include/asm/maccess.h
@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_S390_MACCESS_H
+#define __ASM_S390_MACCESS_H
+
+#include <linux/types.h>
+
+struct iov_iter;
+
+extern unsigned long __memcpy_real_area;
+void memcpy_real_init(void);
+size_t memcpy_real_iter(struct iov_iter *iter, unsigned long src, size_t count);
+int memcpy_real(void *dest, unsigned long src, size_t count);
+
+#endif /* __ASM_S390_MACCESS_H */