[PATCH] s390/kvm: avoid global config of vm.alloc_pgste=1

From: Martin Schwidefsky
Date: Wed Jun 07 2017 - 08:10:24 EST

The system control vm.alloc_pgste is used to control the size of the
page tables, either 2K or 4K. The idea is that a KVM host sets the
vm.alloc_pgste control to 1 which causes *all* new processes to run
with 4K page tables. For a non-kvm system the control should stay off
to save on memory used for page tables.

Trouble is that distributions choose to set the control globally to
be able to run KVM guests. This wastes memory on non-KVM systems.

Introduce the PT_S390_REQUEST_PGSTE ELF segment type to "mark" the
qemu executable with it. All executables with this (empty) segment in
its ELF phdr array will be started with 4K page tables. Any executable
without PT_S390_REQUEST_PGSTE will run with the default 2K page tables.

This removes the need to set vm.alloc_pgste=1 for a KVM host and
minimizes the waste of memory for page tables.

Signed-off-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
---
arch/s390/Kconfig | 1 +
arch/s390/include/asm/elf.h | 26 ++++++++++++++++++++++++++
arch/s390/include/asm/mmu_context.h | 3 ++-
arch/s390/include/asm/ptrace.h | 2 ++
arch/s390/include/asm/thread_info.h | 1 +
arch/s390/kernel/entry.S | 13 ++++++++++++-
6 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 69a77eecaec1..7bd182676ddd 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -64,6 +64,7 @@ config ARCH_SUPPORTS_UPROBES

config S390
def_bool y
+ select ARCH_BINFMT_ELF_STATE
select ARCH_HAS_DEVMEM_IS_ALLOWED
select ARCH_HAS_ELF_RANDOMIZE
select ARCH_HAS_GCOV_PROFILE_ALL
diff --git a/arch/s390/include/asm/elf.h b/arch/s390/include/asm/elf.h
index e8f623041769..7811d1006642 100644
--- a/arch/s390/include/asm/elf.h
+++ b/arch/s390/include/asm/elf.h
@@ -117,6 +117,9 @@
#define ELF_DATA ELFDATA2MSB
#define ELF_ARCH EM_S390

+/* s390 specific phdr types */
+#define PT_S390_REQUEST_PGSTE 0x70000000
+
/*
* ELF register definitions..
*/
@@ -151,6 +154,29 @@ extern unsigned int vdso_enabled;
&& (x)->e_ident[EI_CLASS] == ELF_CLASS)
#define compat_start_thread start_thread31

+struct arch_elf_state {
+};
+
+#define INIT_ARCH_ELF_STATE { }
+
+#define arch_check_elf(ehdr, interp, interp_ehdr, state) (0)
+#define arch_elf_pt_proc(ehdr, phdr, elf, interp, state) \
+({ \
+ struct elf64_hdr *_ehdr = (void *) ehdr; \
+ struct elf64_phdr *_phdr = (void *) phdr; \
+ int _rc = 0; \
+ if (_ehdr->e_ident[EI_CLASS] == ELFCLASS64 && \
+ _phdr->p_type == PT_S390_REQUEST_PGSTE && \
+ !page_table_allocate_pgste && \
+ !test_thread_flag(TIF_REQUEST_PGSTE)) { \
+ set_thread_flag(TIF_REQUEST_PGSTE); \
+ set_pt_regs_flag(task_pt_regs(current), \
+ PIF_SYSCALL_RESTART); \
+ _rc = -EAGAIN; \
+ } \
+ _rc; \
+})
+
/* For SVR4/S390 the function pointer to be registered with `atexit` is
passed in R14. */
#define ELF_PLAT_INIT(_r, load_addr) \
diff --git a/arch/s390/include/asm/mmu_context.h b/arch/s390/include/asm/mmu_context.h
index c119d564d8f2..1201b18e817d 100644
--- a/arch/s390/include/asm/mmu_context.h
+++ b/arch/s390/include/asm/mmu_context.h
@@ -25,7 +25,8 @@ static inline int init_new_context(struct task_struct *tsk,
mm->context.gmap_asce = 0;
mm->context.flush_mm = 0;
#ifdef CONFIG_PGSTE
- mm->context.alloc_pgste = page_table_allocate_pgste;
+ mm->context.alloc_pgste = page_table_allocate_pgste ||
+ test_thread_flag(TIF_REQUEST_PGSTE);
mm->context.has_pgste = 0;
mm->context.use_skey = 0;
mm->context.use_cmma = 0;
diff --git a/arch/s390/include/asm/ptrace.h b/arch/s390/include/asm/ptrace.h
index 99bc456cc26a..24baa80f7af6 100644
--- a/arch/s390/include/asm/ptrace.h
+++ b/arch/s390/include/asm/ptrace.h
@@ -11,9 +11,11 @@

#define PIF_SYSCALL 0 /* inside a system call */
#define PIF_PER_TRAP 1 /* deliver sigtrap on return to user */
+#define PIF_SYSCALL_RESTART 2 /* restart the current system call */

#define _PIF_SYSCALL _BITUL(PIF_SYSCALL)
#define _PIF_PER_TRAP _BITUL(PIF_PER_TRAP)
+#define _PIF_SYSCALL_RESTART _BITUL(PIF_SYSCALL_RESTART)

#ifndef __ASSEMBLY__

diff --git a/arch/s390/include/asm/thread_info.h b/arch/s390/include/asm/thread_info.h
index f36e6e2b73f0..e7444d76cc63 100644
--- a/arch/s390/include/asm/thread_info.h
+++ b/arch/s390/include/asm/thread_info.h
@@ -56,6 +56,7 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src);
#define TIF_NEED_RESCHED 2 /* rescheduling necessary */
#define TIF_UPROBE 3 /* breakpointed or single-stepping */
#define TIF_GUARDED_STORAGE 4 /* load guarded storage control block */
+#define TIF_REQUEST_PGSTE 5 /* New mm's will use 4K page tables */
#define TIF_SYSCALL_TRACE 8 /* syscall trace active */
#define TIF_SYSCALL_AUDIT 9 /* syscall auditing active */
#define TIF_SECCOMP 10 /* secure computing */
diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S
index 0c2c3b8bfc9a..8c824b32527a 100644
--- a/arch/s390/kernel/entry.S
+++ b/arch/s390/kernel/entry.S
@@ -52,7 +52,7 @@ _TIF_TRACE = (_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT | _TIF_SECCOMP | \
_TIF_SYSCALL_TRACEPOINT)
_CIF_WORK = (_CIF_MCCK_PENDING | _CIF_ASCE_PRIMARY | \
_CIF_ASCE_SECONDARY | _CIF_FPU)
-_PIF_WORK = (_PIF_PER_TRAP)
+_PIF_WORK = (_PIF_PER_TRAP | _PIF_SYSCALL_RESTART)

#define BASED(name) name-cleanup_critical(%r13)

@@ -342,6 +342,8 @@ ENTRY(system_call)
jo .Lsysc_guarded_storage
TSTMSK __PT_FLAGS(%r11),_PIF_PER_TRAP
jo .Lsysc_singlestep
+ TSTMSK __PT_FLAGS(%r11),_PIF_SYSCALL_RESTART
+ jo .Lsysc_syscall_restart
TSTMSK __TI_flags(%r12),_TIF_SIGPENDING
jo .Lsysc_sigpending
TSTMSK __TI_flags(%r12),_TIF_NOTIFY_RESUME
@@ -434,6 +436,15 @@ ENTRY(system_call)
jg do_per_trap

#
+# _PIF_SYSCALL_RESTART is set, repeat the current system call
+#
+.Lsysc_syscall_restart:
+ ni __PT_FLAGS+7(%r11),255-_PIF_SYSCALL_RESTART
+ lmg %r1,%r7,__PT_R1(%r11) # load svc arguments
+ lg %r2,__PT_ORIG_GPR2(%r11)
+ j .Lsysc_do_svc
+
+#
# call tracehook_report_syscall_entry/tracehook_report_syscall_exit before
# and after the system call
#
--
2.11.2


--
blue skies,
Martin.

"Reality continues to ruin my life." - Calvin.