Enable "fast system calls" via the 'syscall' instruction on OSv. The
instruction is used by Go programs on Linux/x86-64 for system calls.
Signed-off-by: Pekka Enberg <penb...@scylladb.com>
Signed-off-by: Benoît Canet <ben...@sylladb.com>
---
arch/x64/arch-setup.cc | 12 ++++++++++++
arch/x64/entry.S | 20 ++++++++++++++++++++
arch/x64/msr.hh | 3 +++
linux.cc | 9 +++++++++
4 files changed, 44 insertions(+)
diff --git a/arch/x64/arch-setup.cc b/arch/x64/arch-setup.cc
index 5e76d82..520651d 100644
--- a/arch/x64/arch-setup.cc
+++ b/arch/x64/arch-setup.cc
@@ -10,6 +10,7 @@
#include <osv/mempool.hh>
#include <osv/mmu.hh>
#include "processor.hh"
+#include "processor-flags.h"
#include "msr.hh"
#include "xen.hh"
#include <osv/elf.hh>
@@ -213,6 +214,16 @@ static inline void disable_pic()
XENPV_ALTERNATIVE({ processor::outb(0xff, 0x21); processor::outb(0xff,
0xa1); }, {});
}
+extern "C" void syscall_entry(void);
+
+static void setup_syscall()
+{
+ processor::wrmsr(msr::IA32_STAR, static_cast<uint64_t>(1*8) << 32);
+ processor::wrmsr(msr::IA32_LSTAR,
reinterpret_cast<uint64_t>(syscall_entry));
+ processor::wrmsr(msr::IA32_FMASK, 0);
+ processor::wrmsr(msr::IA32_EFER, processor::rdmsr(msr::IA32_EFER) | 0x01);
+}
+
void arch_init_premain()
{
auto omb = *osv_multiboot_info;
@@ -220,6 +231,7 @@ void arch_init_premain()
debug_early_u64("Error reading disk (real mode): ",
static_cast<u64>(omb.disk_err));
disable_pic();
+ setup_syscall();
}
#include "drivers/driver.hh"
diff --git a/arch/x64/entry.S b/arch/x64/entry.S
index b6f5abe..d3a864a 100644
--- a/arch/x64/entry.S
+++ b/arch/x64/entry.S
@@ -159,3 +159,23 @@ call_signal_handler_thunk:
iretq
.cfi_endproc
+.global syscall_entry
+syscall_entry:
+ # There is no ring transition and rflags are left unchanged. The only
+ # thing we need to save is the rip which is stored in rcx by the syscall
+ # instruction.
+ push %rcx
+ # FIXME: registers clobbered?
+ # FIXME: FPU state?
+ # FIXME: system call arguments?
+ movq %r10, %rcx
+ # rotate syscall arguments
+ movq %r8, %r9
+ movq %rcx, %r8
+ movq %rdx, %rcx
+ movq %rsi, %rdx
+ movq %rdi, %rsi
+ movq %rax, %rdi
+ call syscall_wrapper
+ pop %rcx
+ jmp *%rcx
diff --git a/arch/x64/msr.hh b/arch/x64/msr.hh
index 154bba7..d77c75c 100644
--- a/arch/x64/msr.hh
+++ b/arch/x64/msr.hh
@@ -58,6 +58,9 @@ enum class msr : uint32_t {
IA32_APIC_BASE = 0x0000001b,
IA32_EFER = 0xc0000080,
+ IA32_STAR = 0xc0000081,
+ IA32_LSTAR = 0xc0000082,
+ IA32_FMASK = 0xc0000084,
IA32_FS_BASE = 0xc0000100,
KVM_WALL_CLOCK = 0x11,
diff --git a/linux.cc b/linux.cc
index bd82ca9..8a2a4a3 100644
--- a/linux.cc
+++ b/linux.cc
@@ -291,3 +291,12 @@ long syscall(long number, ...)
return -1;
}
long __syscall(long number, ...) __attribute__((alias("syscall")));
+
+extern "C" long syscall_wrapper(long number, ...)
+{
+ auto ret = syscall(number);
+ if (ret < 0) {
+ return -errno;
+ }
+ return 0;
+}
--
2.7.4
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