From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeremy Fitzhardinge Subject: [patch 32/44] xen: hack to prevent bad segment register reload Date: Mon, 16 Jul 2007 16:16:08 -0700 Message-ID: <45830.350140068$1184631235@news.gmane.org> References: <20070716231536.937393000@xensource.com>> Return-path: Content-Disposition: inline; filename=xen-segreg-hack.patch List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Linus Torvalds Cc: Jeremy Fitzhardinge , Xen-devel , Andi Kleen , lkml , Chris Wright , Andrew Morton List-Id: xen-devel@lists.xenproject.org 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 Signed-off-by: Chris Wright --- 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, --