Changeset 1a5eca4 in mainline for uspace/lib/c/arch/amd64/src/fibril.S

Timestamp:

2016-04-27T19:36:56Z (8 years ago)

Author:

Jakub Jermar <jakub@…>

Branches:

lfn, master, serial, ticket/834-toolchain-update, topic/msim-upgrade, topic/simplify-dev-export

Children:

af9dd1e

Parents:

d6f9fff

Message:

amd64: Make TLS settable from uspace

The TLS document[1] mandates that %fs[0] is the thread pointer on amd64.
That is good as it allows userspace-only TLS management for fibrils:
fibril_save/restore() simply manipulate the thread pointer in %fs:0 and
don't need to ask the kernel to modify %fs's base. The kernel treats
%fs:0 as another preserved register and preserves it across context
switches. GCC gets in the way a little bit because it by default assumes
that TLS is accessible from negative %fs offsets (which would
necessitate a kernel-assisted solution). Fortunately, there is a GCC
option to suppress this assumption.

• Introduce the concept of virtual registers, with VREG_TP (thread pointer) being the first of them
• Preserve VREG_TP in context_save/restore()
• Stop using sys_tls_set() in favour of using %fs:0 as the thread pointer
• Make GCC generate code that always goes through %gs:0 to access TLS
• Introduce kseg: a per-CPU area accessible from GS_KERNEL that holds the kernel stack, kernel FS base and a scratch space for syscall and int handlers to use

[1] Drepper, U.: ELF Handling For Thread-Local Storage

File:

• uspace/lib/c/arch/amd64/src/fibril.S (modified) (2 diffs)

uspace/lib/c/arch/amd64/src/fibril.S

@@ -51 +51 @@
         movq %r15, CONTEXT_OFFSET_R15(%rdi)
         
-        # save TLS
         movq %fs:0, %rax
         movq %rax, CONTEXT_OFFSET_TLS(%rdi)
@@ -79 +78 @@
         movq %rdx,(%rsp)
         
-        # Set thread local storage
-        movq CONTEXT_OFFSET_TLS(%rdi), %rdi  # Set arg1 to TLS addr
-        movl $1, %eax                        # SYS_TLS_SET
-        syscall
+        movq CONTEXT_OFFSET_TLS(%rdi), %rdi
+        movq %rdi, %fs:0
         
         xorl %eax, %eax                      # context_restore returns 0