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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Jul 17, 2015 at 11:36:29AM -0400, Boris Ostrovsky wrote:
> 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?

I was surprised as well - I was expecting your patch to have blow up.
Unless we are doing something fancy for CPU0 and for the other CPUs we
already have the per-cpu segment setup during bootup (copied from BSP)?

> 
> >
> >>+           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.

Right, but it uses the per-cpu area so I would have expected it to crash.
(since you did not do load_percpu_segment).But maybe it did load, but some
unknown area ? :-)
> 
> -boris
> 
> >>            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);
> >>  #else
> >>-           /* 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);
> >>  #endif
> >>            return; /* PVH does not need any PV GDT ops. */
> >>    }
> >>-- 
> >>1.8.1.4
> >>
> 

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