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 11:36:29 -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 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?

>
>> +		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.

-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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