Checkpoint some of what I was starting to tinker with for having some

different context support for 32 vs 64 bit processes.  This simply omits
the save/restore of the segment selector registers for non 32 bit
processes.  This avoids the rdmsr/rwmsr juggling when restoring %gs
clobbers the kernel msr that holds the gsbase.

However, I suspect it might be better to conditionally do this at
user<->kernel transition where we wouldn't need to do the juggling in the
first place.  Or have per-thread extended context save/restore hooks.

3 files changed, 20 insertions, 7 deletions

diff --git a/sys/amd64/amd64/cpu_switch.S b/sys/amd64/amd64/cpu_switch.S
index 2bbd904..ae13f58 100644
--- a/sys/amd64/amd64/cpu_switch.S
+++ b/sys/amd64/amd64/cpu_switch.S
@@ -103,6 +103,17 @@ ENTRY(cpu_switch)
 	pushfq					/* PSL */
 	popq	PCB_RFLAGS(%r8)
 
+	testl	$PCB_32BIT,PCB_FLAGS(%r8)
+	jz	1f				/* no, skip over */
+
+	/* Save segment selector numbers */
+	movl	%ds,PCB_DS(%r8)
+	movl	%es,PCB_ES(%r8)
+	movl	%fs,PCB_FS(%r8)
+	movl	%gs,PCB_GS(%r8)
+	jmp	2f
+1:
+
 	/* Save userland %fs */
 	movl	$MSR_FSBASE,%ecx
 	rdmsr
@@ -114,12 +125,7 @@ ENTRY(cpu_switch)
 	rdmsr
 	movl	%eax,PCB_GSBASE(%r8)
 	movl	%edx,PCB_GSBASE+4(%r8)
-
-	/* Save segment selector numbers */
-	movl	%ds,PCB_DS(%r8)
-	movl	%es,PCB_ES(%r8)
-	movl	%fs,PCB_FS(%r8)
-	movl	%gs,PCB_GS(%r8)
+2:
 
 	/* Test if debug registers should be saved. */
 	testl	$PCB_DBREGS,PCB_FLAGS(%r8)
@@ -181,6 +187,9 @@ sw1:
 	 */
 	movq	TD_PCB(%rsi),%r8
 
+	testl	$PCB_32BIT,PCB_FLAGS(%r8)
+	jz	1f				/* no, skip over */
+
 	/* Restore segment selector numbers */
 	movl	PCB_DS(%r8),%ds
 	movl	PCB_ES(%r8),%es
@@ -191,6 +200,8 @@ sw1:
 	rdmsr
 	movl	PCB_GS(%r8),%gs
 	wrmsr
+	jmp	2f
+1:
 
 	/* Restore userland %fs */
 	movl	$MSR_FSBASE,%ecx
@@ -203,6 +214,7 @@ sw1:
 	movl	PCB_GSBASE(%r8),%eax
 	movl	PCB_GSBASE+4(%r8),%edx
 	wrmsr
+2:
 
 	/* Update the TSS_RSP0 pointer for the next interrupt */
 	movq	PCPU(TSSP), %rax
@@ -248,7 +260,6 @@ sw1:
 	orq	%rcx,%rax
 	movq	%rax,%dr7
 1:
-
 	ret
 
 /*
diff --git a/sys/amd64/amd64/genassym.c b/sys/amd64/amd64/genassym.c
index 933c7be..5152be5 100644
--- a/sys/amd64/amd64/genassym.c
+++ b/sys/amd64/amd64/genassym.c
@@ -138,6 +138,7 @@ ASSYM(PCB_DR3, offsetof(struct pcb, pcb_dr3));
 ASSYM(PCB_DR6, offsetof(struct pcb, pcb_dr6));
 ASSYM(PCB_DR7, offsetof(struct pcb, pcb_dr7));
 ASSYM(PCB_DBREGS, PCB_DBREGS);
+ASSYM(PCB_32BIT, PCB_32BIT);
 
 ASSYM(PCB_FLAGS, offsetof(struct pcb, pcb_flags));
 ASSYM(PCB_FULLCTX, PCB_FULLCTX);
diff --git a/sys/amd64/include/pcb.h b/sys/amd64/include/pcb.h
index de9bad8..73d8aa5 100644
--- a/sys/amd64/include/pcb.h
+++ b/sys/amd64/include/pcb.h
@@ -71,6 +71,7 @@ struct pcb {
 	u_long	pcb_flags;
 #define	PCB_DBREGS	0x02	/* process using debug registers */
 #define	PCB_FPUINITDONE	0x08	/* fpu state is initialized */
+#define	PCB_32BIT	0x40	/* process has 32 bit context (segs etc) */
 #define	PCB_FULLCTX	0x80	/* full context restore on sysret */
 
 	caddr_t	pcb_onfault;	/* copyin/out fault recovery */