[Xen-devel] [PATCH 28 of 36] Save %fs and %gs before load_TLS() and arch_leave_lazy_cpu_mode()

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

    We must do this because load_TLS() may need to clear %fs and %gs,
    such (e.g. under Xen).

Signed-off-by: Eduardo Habkost <ehabkost@xxxxxxxxxx>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
---
 arch/x86/kernel/process_64.c |   17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -579,6 +579,7 @@
                                 *next = &next_p->thread;
        int cpu = smp_processor_id();
        struct tss_struct *tss = &per_cpu(init_tss, cpu);
+       unsigned fsindex, gsindex;
 
        /* we're going to use this soon, after a few expensive things */
        if (next_p->fpu_counter>5)
@@ -601,6 +602,15 @@
        if (unlikely(next->ds | prev->ds))
                loadsegment(ds, next->ds);
 
+
+       /* We must save %fs and %gs before load_TLS() because
+        * %fs and %gs may be cleared by load_TLS().
+        *
+        * (e.g. xen_load_tls())
+        */
+       savesegment(fs, fsindex);
+       savesegment(gs, gsindex);
+
        load_TLS(next, cpu);
 
        /*
@@ -616,8 +626,6 @@
         * Switch FS and GS.
         */
        { 
-               unsigned fsindex;
-               savesegment(fs, fsindex);
                /* segment register != 0 always requires a reload. 
                   also reload when it has changed. 
                   when prev process used 64bit base always reload
@@ -635,10 +643,7 @@
                if (next->fs) 
                        wrmsrl(MSR_FS_BASE, next->fs); 
                prev->fsindex = fsindex;
-       }
-       { 
-               unsigned gsindex;
-               savesegment(gs, gsindex);
+
                if (unlikely(gsindex | next->gsindex | prev->gs)) {
                        load_gs_index(next->gsindex);
                        if (gsindex)



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