WARNING - OLD ARCHIVES

This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-devel

[Xen-devel] [patch 24/33] xen: xen: hack to prevent bad segment register

To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Andi Kleen <ak@xxxxxxx>
Subject: [Xen-devel] [patch 24/33] xen: xen: hack to prevent bad segment register reload
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Tue, 22 May 2007 15:10:05 +0100
Cc: Chris Wright <chrisw@xxxxxxxxxxxx>, virtualization@xxxxxxxxxxxxxx, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, lkml <linux-kernel@xxxxxxxxxxxxxxx>
Delivery-date: Tue, 22 May 2007 07:54:13 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxx
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
References: <20070522140941.802382212@xxxxxxxx>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: quilt/0.46-1
The hypervisor saves and restores the segment registers as part of the
state is saves while context switching.  If, during a context switch,
the next process doesn't use the TLS segments, it invalidates the GDT
entry, causing the segment register reload to fault.  This fault
effectively doubles the cost of a context switch.

This patch is a band-aid workaround which clears the usermode %gs
after it has been saved for the previous process, but before it gets
reloaded for the next, and it avoids having the hypervisor attempt to
erroneously reload it.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx>

---
 arch/i386/xen/enlighten.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

===================================================================
--- a/arch/i386/xen/enlighten.c
+++ b/arch/i386/xen/enlighten.c
@@ -291,6 +291,18 @@ static void xen_load_tls(struct thread_s
        load_TLS_descriptor(t, cpu, 2);
 
        xen_mc_issue(PARAVIRT_LAZY_CPU);
+
+       /*
+        * XXX sleazy hack: If we're being called in a lazy-cpu zone,
+        * it means we're in a context switch, and %gs has just been
+        * saved.  This means we can zero it out to prevent faults on
+        * exit from the hypervisor if the next process has no %gs.
+        * Either way, it has been saved, and the new value will get
+        * loaded properly.  This will go away as soon as Xen has been
+        * modified to not save/restore %gs for normal hypercalls.
+        */
+       if (xen_get_lazy_mode() == PARAVIRT_LAZY_CPU)
+               loadsegment(gs, 0);
 }
 
 static void xen_write_ldt_entry(struct desc_struct *dt, int entrynum, u32 low, 
u32 high)

-- 


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel

<Prev in Thread] Current Thread [Next in Thread>