s390/smp: rework absolute lowcore access

Temporary unsetting of the prefix page in memcpy_absolute() routine
poses a risk of executing code path with unexpectedly disabled prefix
page. This rework avoids the prefix page uninstalling and disabling
of normal and machine check interrupts when accessing the absolute
zero memory.

Although memcpy_absolute() routine can access the whole memory, it is
only used to update the absolute zero lowcore. This rework therefore
introduces a new mechanism for the absolute zero lowcore access and
scraps memcpy_absolute() routine for good.

Instead, an area is reserved in the virtual memory that is used for
the absolute lowcore access only. That area holds an array of 8KB
virtual mappings - one per CPU. Whenever a CPU is brought online, the
corresponding item is mapped to the real address of the previously
installed prefix page.

The absolute zero lowcore access works like this: a CPU calls the
new primitive get_abs_lowcore() to obtain its 8KB mapping as a
pointer to the struct lowcore. Virtual address references to that
pointer get translated to the real addresses of the prefix page,
which in turn gets swapped with the absolute zero memory addresses
due to prefixing. Once the pointer is not needed it must be released
with put_abs_lowcore() primitive:

	struct lowcore *abs_lc;
	unsigned long flags;

	abs_lc = get_abs_lowcore(&flags);
	abs_lc->... = ...;
	put_abs_lowcore(abs_lc, flags);

To ensure the described mechanism works large segment- and region-
table entries must be avoided for the 8KB mappings. Failure to do
so results in usage of Region-Frame Absolute Address (RFAA) or
Segment-Frame Absolute Address (SFAA) large page fields. In that
case absolute addresses would be used to address the prefix page
instead of the real ones and the prefixing would get bypassed.

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

3 files changed, 19 insertions, 15 deletions

diff --git a/arch/s390/include/asm/abs_lowcore.h b/arch/s390/include/asm/abs_lowcore.h
new file mode 100644
index 000000000000..bdef8d24d237
--- /dev/null
+++ b/arch/s390/include/asm/abs_lowcore.h
@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_S390_ABS_LOWCORE_H
+#define _ASM_S390_ABS_LOWCORE_H
+
+#include <asm/lowcore.h>
+
+#define ABS_LOWCORE_MAP_SIZE	(NR_CPUS * sizeof(struct lowcore))
+
+extern unsigned long __abs_lowcore;
+extern bool abs_lowcore_mapped;
+
+struct lowcore *get_abs_lowcore(unsigned long *flags);
+void put_abs_lowcore(struct lowcore *lc, unsigned long flags);
+int abs_lowcore_map(int cpu, struct lowcore *lc);
+void abs_lowcore_unmap(int cpu);
+
+#endif /* _ASM_ABS_S390_LOWCORE_H */
diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
index a397b072a580..82506ebd544b 100644
--- a/arch/s390/include/asm/pgtable.h
+++ b/arch/s390/include/asm/pgtable.h
@@ -1781,6 +1781,8 @@ static inline swp_entry_t __swp_entry(unsigned long type, unsigned long offset)
 
 extern int vmem_add_mapping(unsigned long start, unsigned long size);
 extern void vmem_remove_mapping(unsigned long start, unsigned long size);
+extern int vmem_map_4k_page(unsigned long addr, unsigned long phys, pgprot_t prot);
+extern void vmem_unmap_4k_page(unsigned long addr);
 extern int s390_enable_sie(void);
 extern int s390_enable_skey(void);
 extern void s390_reset_cmma(struct mm_struct *mm);
diff --git a/arch/s390/include/asm/processor.h b/arch/s390/include/asm/processor.h
index bd66f8e34949..93677ae89e7e 100644
--- a/arch/s390/include/asm/processor.h
+++ b/arch/s390/include/asm/processor.h
@@ -307,21 +307,6 @@ static __always_inline void __noreturn disabled_wait(void)
 #define ARCH_LOW_ADDRESS_LIMIT	0x7fffffffUL
 
 extern int memcpy_real(void *, unsigned long, size_t);
-extern void memcpy_absolute(void *, void *, size_t);
-
-#define put_abs_lowcore(member, x) do {					\
-	unsigned long __abs_address = offsetof(struct lowcore, member);	\
-	__typeof__(((struct lowcore *)0)->member) __tmp = (x);		\
-									\
-	memcpy_absolute(__va(__abs_address), &__tmp, sizeof(__tmp));	\
-} while (0)
-
-#define get_abs_lowcore(x, member) do {					\
-	unsigned long __abs_address = offsetof(struct lowcore, member);	\
-	__typeof__(((struct lowcore *)0)->member) *__ptr = &(x);	\
-									\
-	memcpy_absolute(__ptr, __va(__abs_address), sizeof(*__ptr));	\
-} while (0)
 
 extern int s390_isolate_bp(void);
 extern int s390_isolate_bp_guest(void);