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Re: [Xen-devel] [PATCH v2 4/6] xen/x86/pvh: Set up descriptors for 32-bit PVH guests

On 07/17/2015 11:21 AM, Konrad Rzeszutek Wilk wrote:
On Thu, Jul 16, 2015 at 05:43:39PM -0400, Boris Ostrovsky wrote:
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Changes in v2:
* Set segment selectors using loadsegment() instead of assembly

  arch/x86/xen/enlighten.c | 15 ++++++++++-----
  1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index f8dc398..d665b1d 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -1362,12 +1362,12 @@ static void __init xen_boot_params_init_edd(void)
  static void __ref xen_setup_gdt(int cpu)
        if (xen_feature(XENFEAT_auto_translated_physmap)) {
-#ifdef CONFIG_X86_64
-               unsigned long dummy;
+               unsigned long __attribute__((unused)) dummy;
- load_percpu_segment(cpu); /* We need to access per-cpu area */
You removed that - where are we going to do that? As the
'switch_to_new_gdt' uses the per-cpu GDT table.

load_percpu_segment() is part of switch_to_new_gdt(), so I thought there is no need to call it here.

But you are right --- switch_to_new_gdt() starts with get_cpu_gdt_table() which accesses per-CPU area. How did this manage to work then?

+               setup_stack_canary_segment(cpu);
As this one too ?

This one I added. On 64-bit it's a nop so we never had problems without having this call. On 32-bit, if CC_STACKPROTECTOR is set, we will fail without setting this up.


                switch_to_new_gdt(cpu); /* GDT and GS set */
+#ifdef CONFIG_X86_64
                /* We are switching of the Xen provided GDT to our HVM mode
                 * GDT. The new GDT has  __KERNEL_CS with CS.L = 1
                 * and we are jumping to reload it.
@@ -1392,8 +1392,13 @@ static void __ref xen_setup_gdt(int cpu)
                loadsegment(ds, 0);
                loadsegment(fs, 0);
-               /* PVH: TODO Implement. */
-               BUG();
+               asm volatile ("ljmp %0,$1f\n"
+                             "1:\n"
+                             :: "i" (__KERNEL_CS));
+               loadsegment(ss, __KERNEL_DS);
+               loadsegment(ds, __USER_DS);
+               loadsegment(es, __USER_DS);
                return; /* PVH does not need any PV GDT ops. */

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