Re: [PATCH 3/3] Eliminate read_cr3 on TLB flush

[Anthony Liguori <aliguori@us.ibm.com>]

[-- Attachment #1: Type: text/plain, Size: 576 bytes --]

Andi Kleen wrote:
> On Wednesday 30 May 2007 16:53:41 Anthony Liguori wrote:
>   
>> Subject: [PATCH][PARAVIRT] Eliminate unnecessary CR3 read in TLB flush
>>
>> This patch eliminates the CR3 read (which would cause a VM exit) in the TLB
>> flush path.  The patch is based on Ingo Molnar's paravirt series.
>>
>>     
>
> This change could be just done generically for the native architecture, couldn't 
> it?
>   

Sure.  It adds a few more cycles onto native though (two memory reads, 
and some math).  How does the following look?

Regards,

Anthony Liguori

> -Andi
>   


[-- Attachment #2: no-read-cr3-on-tlbflush.diff --]
[-- Type: text/x-patch, Size: 2122 bytes --]

Subject: [PATCH] Avoid reading CR3 on TLB flush
From: Anthony Liguori <aliguori@us.ibm.com>

In a virtualized environment, there is significant overhead in reading and
writing control registers.  Since we already have the value of CR3 in
current->active_mm->pgd, we can avoid taking the exit on CR3 read when
doing a TLB flush.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

Index: kvm/include/asm-i386/tlbflush.h
===================================================================
--- kvm.orig/include/asm-i386/tlbflush.h	2007-05-30 10:22:48.000000000 -0500
+++ kvm/include/asm-i386/tlbflush.h	2007-05-30 10:22:54.000000000 -0500
@@ -14,13 +14,12 @@
 
 #define __native_flush_tlb()						\
 	do {								\
-		unsigned int tmpreg;					\
+		unsigned int cr3 = __pa(current->active_mm->pgd);	\
 									\
 		__asm__ __volatile__(					\
-			"movl %%cr3, %0;              \n"		\
 			"movl %0, %%cr3;  # flush TLB \n"		\
-			: "=r" (tmpreg)					\
-			:: "memory");					\
+			:: "r" (cr3)				       	\
+			: "memory");					\
 	} while (0)
 
 /*
@@ -29,18 +28,18 @@
  */
 #define __native_flush_tlb_global()					\
 	do {								\
-		unsigned int tmpreg, cr4, cr4_orig;			\
+		unsigned int cr3 = __pa(current->active_mm->pgd);	\
+		unsigned int cr4, cr4_orig;				\
 									\
 		__asm__ __volatile__(					\
-			"movl %%cr4, %2;  # turn off PGE     \n"	\
-			"movl %2, %1;                        \n"	\
-			"andl %3, %1;                        \n"	\
-			"movl %1, %%cr4;                     \n"	\
-			"movl %%cr3, %0;                     \n"	\
-			"movl %0, %%cr3;  # flush TLB        \n"	\
-			"movl %2, %%cr4;  # turn PGE back on \n"	\
-			: "=&r" (tmpreg), "=&r" (cr4), "=&r" (cr4_orig)	\
-			: "i" (~X86_CR4_PGE)				\
+			"movl %%cr4, %1;  # turn off PGE     \n"	\
+			"movl %1, %0;                        \n"	\
+			"andl %2, %0;                        \n"	\
+			"movl %0, %%cr4;                     \n"	\
+			"movl %3, %%cr3;  # flush TLB        \n"	\
+			"movl %1, %%cr4;  # turn PGE back on \n"	\
+			: "=&r" (cr4), "=&r" (cr4_orig)			\
+			: "i" (~X86_CR4_PGE), "r" (cr3)			\
 			: "memory");					\
 	} while (0)
 

[-- Attachment #3: Type: text/plain, Size: 184 bytes --]

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linux-foundation.org/mailman/listinfo/virtualization