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

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Date:	Fri, 17 Jul 2015 13:52:12 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	david.vrabel@...rix.com, roger.pau@...rix.com,
	elena.ufimtseva@...cle.com, stefano.stabellini@...citrix.com,
	tim@....org, jbeulich@...e.com, andrew.cooper3@...rix.com,
	ian.campbell@...rix.com, ian.jackson@...citrix.com,
	xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/6] xen/x86/pvh: Set up descriptors for 32-bit PVH
 guests

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@...cle.com>
>>>> ---
>>>> 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.

-boris

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