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Jeremy Fitzhardinge

2007-May-21 09:28 UTC

changing definition of paravirt_ops.iret

I'm implementing a more efficient version of the Xen iret paravirt_op,
so that it can use the real iret instruction where possible.  I really
need to get access to per-cpu variables, so I can set the event mask
state in the vcpu_info structure, but unfortunately at the point where
INTERRUPT_RETURN is used in entry.S, the usermode %fs has already been
restored.

How would you feel if we changed paravirt_ops.iret to make it also
responsible for restoring %fs? 

In other words, change RESTORE_REGS to skip %fs, and then native_iret
would look like:

1:	popl %fs
	iret

with the normal exception stuff.  Fortunately, %fs is already the first
thing to be saved and last to be restored, so there's no major
rearrangements.

Ideally I'd also like a register to play with, but that would require
rearranging pt_regs, which is a bit tricky.

Also, INTERRUPT_RETURN already has a bug, in which it needs to deal with
its own iret exceptions.

    J

diff -r e13ec2ed67aa arch/i386/kernel/entry.S
--- a/arch/i386/kernel/entry.S	Mon May 21 16:56:20 2007 +0100
+++ b/arch/i386/kernel/entry.S	Mon May 21 17:21:37 2007 +0100
@@ -59,6 +59,9 @@
  *   INTERRUPT_RETURN (aka. "iret")
  *   GET_CR0_INTO_EAX (aka. "movl %cr0, %eax")
  *   ENABLE_INTERRUPTS_SYSEXIT (aka "sti; sysexit").
+ *
+ * INTERRUPT_RETURN is responsible for restoring the usermode %fs
+ * from the stack.
  *
  * For DISABLE_INTERRUPTS/ENABLE_INTERRUPTS (aka
"cli"/"sti"), you must
  * specify what registers can be overwritten (CLBR_NONE, CLBR_EAX/EDX/ECX/ANY).
@@ -166,21 +169,15 @@ 2:	popl %es;	\
 2:	popl %es;	\
 	CFI_ADJUST_CFA_OFFSET -4;\
 	/*CFI_RESTORE es;*/\
-3:	popl %fs;	\
-	CFI_ADJUST_CFA_OFFSET -4;\
-	/*CFI_RESTORE fs;*/\
 .pushsection .fixup,"ax";	\
 4:	movl $0,(%esp);	\
 	jmp 1b;		\
 5:	movl $0,(%esp);	\
 	jmp 2b;		\
-6:	movl $0,(%esp);	\
-	jmp 3b;		\
 .section __ex_table,"a";\
 	.align 4;	\
 	.long 1b,4b;	\
 	.long 2b,5b;	\
-	.long 3b,6b;	\
 .popsection
 
 #define RING0_INT_FRAME \
@@ -406,19 +403,14 @@ restore_nocheck_notrace:
 	RESTORE_REGS
 	addl $4, %esp			# skip orig_eax/error_code
 	CFI_ADJUST_CFA_OFFSET -4
-1:	INTERRUPT_RETURN
-.section .fixup,"ax"
+	INTERRUPT_RETURN
+
 iret_exc:
 	TRACE_IRQS_ON
 	ENABLE_INTERRUPTS(CLBR_NONE)
 	pushl $0			# no error code
 	pushl $do_iret_error
 	jmp error_code
-.previous
-.section __ex_table,"a"
-	.align 4
-	.long 1b,iret_exc
-.previous
 
 	CFI_RESTORE_STATE
 ldt_ss:
@@ -863,21 +855,22 @@ nmi_espfix_stack:
 	RESTORE_REGS
 	lss 12+4(%esp), %esp		# back to espfix stack
 	CFI_ADJUST_CFA_OFFSET -24
-1:	INTERRUPT_RETURN
-	CFI_ENDPROC
+	INTERRUPT_RETURN
+	CFI_ENDPROC
+KPROBE_END(nmi)
+
+#ifdef CONFIG_PARAVIRT
+ENTRY(native_iret)
+1:	popl %fs
+2:	iret
+.pushsection .fixup,"ax"
+3:	movl $0,(%esp)
+	jmp 1b
 .section __ex_table,"a"
 	.align 4
-	.long 1b,iret_exc
-.previous
-KPROBE_END(nmi)
-
-#ifdef CONFIG_PARAVIRT
-ENTRY(native_iret)
-1:	iret
-.section __ex_table,"a"
-	.align 4
-	.long 1b,iret_exc
-.previous
+	.long 1b,3b
+	.long 2b,iret_exc
+.popsection
 END(native_iret)
 
 ENTRY(native_irq_enable_sysexit)
diff -r e13ec2ed67aa arch/i386/xen/enlighten.c
--- a/arch/i386/xen/enlighten.c	Mon May 21 16:56:20 2007 +0100
+++ b/arch/i386/xen/enlighten.c	Mon May 21 17:21:37 2007 +0100
@@ -851,7 +851,6 @@ static unsigned xen_patch(u8 type, u16 c
 		SITE(irq_disable);
 		SITE(save_fl);
 		SITE(restore_fl);
-		SITE(iret);
 #undef SITE
 
 	patch_site:
@@ -935,7 +934,7 @@ static const struct paravirt_ops xen_par
 	.read_tsc = native_read_tsc,
 	.read_pmc = native_read_pmc,
 
-	.iret = (void *)&hypercall_page[__HYPERVISOR_iret],
+	.iret = xen_iret,
 	.irq_enable_sysexit = NULL,  /* never called */
 
 	.load_tr_desc = paravirt_nop,
diff -r e13ec2ed67aa include/asm-i386/irqflags.h
--- a/include/asm-i386/irqflags.h	Mon May 21 16:56:20 2007 +0100
+++ b/include/asm-i386/irqflags.h	Mon May 21 17:21:37 2007 +0100
@@ -106,7 +106,18 @@ static inline unsigned long __raw_local_
 #define DISABLE_INTERRUPTS(clobbers)	cli
 #define ENABLE_INTERRUPTS(clobbers)	sti
 #define ENABLE_INTERRUPTS_SYSEXIT	sti; sysexit
-#define INTERRUPT_RETURN		iret
+#define INTERRUPT_RETURN				\
+1:	popl %fs;					\
+2:	iret;						\
+.pushsection .fixup,"ax";				\
+3:	movl $0,(%esp);					\
+	jmp 1b;						\
+.section __ex_table,"a";				\
+	.align 4;					\
+	.long 1b,3b;					\
+	.long 2b,iret_exc;				\
+.popsection
+
 #define GET_CR0_INTO_EAX		movl %cr0, %eax
 #endif /* __ASSEMBLY__ */
 #endif /* CONFIG_PARAVIRT */

Chris Wright

2007-May-21 09:47 UTC

head link

changing definition of paravirt_ops.iret

* Jeremy Fitzhardinge (jeremy@goop.org) wrote:> I'm implementing a more efficient version of the Xen iret paravirt_op,
> so that it can use the real iret instruction where possible.  I really
> need to get access to per-cpu variables, so I can set the event mask
> state in the vcpu_info structure, but unfortunately at the point where
> INTERRUPT_RETURN is used in entry.S, the usermode %fs has already been
> restored.
> 
> How would you feel if we changed paravirt_ops.iret to make it also
> responsible for restoring %fs? 
This is definitely ad-hoc semantic change, but I don't see a beter
way to do it (other than have iret be restore_regs_and_iret, which
isn't really an improvement).

thanks,
-chris

Zachary Amsden

2007-May-21 11:43 UTC

head link

changing definition of paravirt_ops.iret

Jeremy Fitzhardinge wrote:> I'm implementing a more efficient version of the Xen iret paravirt_op,
> so that it can use the real iret instruction where possible.  I really
> need to get access to per-cpu variables, so I can set the event mask
> state in the vcpu_info structure, but unfortunately at the point where
> INTERRUPT_RETURN is used in entry.S, the usermode %fs has already been
> restored.
>
> How would you feel if we changed paravirt_ops.iret to make it also
> responsible for restoring %fs? 
>
> In other words, change RESTORE_REGS to skip %fs, and then native_iret
> would look like:
>
> 1:	popl %fs
> 	iret
>
> with the normal exception stuff.  Fortunately, %fs is already the first
> thing to be saved and last to be restored, so there's no major
> rearrangements.
>
> Ideally I'd also like a register to play with, but that would require
> rearranging pt_regs, which is a bit tricky.
>   
Strongly nacked.  If you need %fs and a free register, just push / pop 
them yourself.  We use push / pop to get %eax free for IRET in the VMI ROM.

A change in IRET call convention is performance critical, and has severe 
negative effects on VMI, since we then need to wrap the IRET entry point 
in the ROM with more code in Linux - so iret is a jump to fs restore, 
call to ROM gobbledygook that misaligns the call/return stack and rots 
performance in some hitherto unkown case - perhaps on native.

There is absolutely no argument that re-arranging entry.S like this is 
nightmarish and makes the code near unmaintainable without carefully 
considering the effects on paravirt-ops hypervisors.

If you really absolutely must avoid reloading %fs for your own 
performance, I can suggest several alternative schemes that, while more 
complex, might also make native exception handling faster, thus 
justifying the hack-to-bitty-pieces of entry.S

Zach

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