Introduce curpcb magic variable, similar to curthread, which is MD

amd64.  It is implemented as __pure2 inline with non-volatile asm read
from pcpu, which allows a compiler to cache its results.

Convert most PCPU_GET(pcb) and curthread->td_pcb accesses into curpcb.

Note that __curthread() uses magic value 0 as an offsetof(struct pcpu,
pc_curthread). It seems to be done this way due to machine/pcpu.h
needs to be processed before sys/pcpu.h, because machine/pcpu.h
contributes machine-depended fields to the struct pcpu definition. As
result, machine/pcpu.h cannot use struct pcpu yet.

The __curpcb() also uses a magic constant instead of offsetof(struct
pcpu, pc_curpcb) for the same reason. The constants are now defined as
symbols and CTASSERTs are added to ensure that future KBI changes do
not break the code.

Requested and reviewed by: bde
MFC after:    3 weeks

1 files changed, 14 insertions, 1 deletions

diff --git a/sys/amd64/include/pcpu.h b/sys/amd64/include/pcpu.h
index d07dbac..5d1fd4d 100644
--- a/sys/amd64/include/pcpu.h
+++ b/sys/amd64/include/pcpu.h
@@ -216,16 +216,29 @@ extern struct pcpu *pcpup;
 #define	PCPU_PTR(member)	__PCPU_PTR(pc_ ## member)
 #define	PCPU_SET(member, val)	__PCPU_SET(pc_ ## member, val)
 
+#define	OFFSETOF_CURTHREAD	0
 static __inline __pure2 struct thread *
 __curthread(void)
 {
 	struct thread *td;
 
-	__asm("movq %%gs:0,%0" : "=r" (td));
+	__asm("movq %%gs:%1,%0" : "=r" (td)
+	    : "m" (*(char *)OFFSETOF_CURTHREAD));
 	return (td);
 }
 #define	curthread		(__curthread())
 
+#define	OFFSETOF_CURPCB		32
+static __inline __pure2 struct pcb *
+__curpcb(void)
+{
+	struct pcb *pcb;
+
+	__asm("movq %%gs:%1,%0" : "=r" (pcb) : "m" (*(char *)OFFSETOF_CURPCB));
+	return (pcb);
+}
+#define	curpcb		(__curpcb())
+
 #define	IS_BSP()	(PCPU_GET(cpuid) == 0)
 
 #else /* !lint || defined(__GNUCLIKE_ASM) && defined(__GNUCLIKE___TYPEOF) */