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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 07/17/2015 01:52 PM, Boris Ostrovsky wrote:
> On 07/17/2015 12:43 PM, Konrad Rzeszutek Wilk wrote:
> > 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)?
>
> No, %fs is zero when we enter xen_setup_gdt() (for 32-bit).
>
> In any case, I should put load_percpu_segment() back.

No, I shouldn't.

Until the new GDT is loaded we can't load selectors since current GDT 
doesn't have descriptors set up for them. And so any attempt to load 
uninitialized selectors results in a fault.
This worked for 64-bit guests because there we load zero into %gs and 
that is allowed (processor doesn't perform descriptor checks for the 
first 4 indexes). But for 32-bit guests we load %fs with 0xd8.
And the reason the code worked before was because we are using "master" 
per-cpu area and because GDT is the same for all CPUs at that point. Or 
so I think.

-boris

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