linux-kernel - Re: [Xen-devel] [PATCH] x86/xen: Fix 32-bit PV guests's usage of kernel

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Message-ID: <53455933.2060406@citrix.com>
Date:	Wed, 9 Apr 2014 15:29:07 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	<boris.ostrovsky@...cle.com>, <xen-devel@...ts.xenproject.org>,
	<konrad.wilk@...cle.com>, <srostedt@...hat.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] x86/xen: Fix 32-bit PV guests's usage of
 kernel_stack

On 09/04/14 15:21, Jan Beulich wrote:
>>>> On 09.04.14 at 16:06, <boris.ostrovsky@...cle.com> wrote:
>> --- a/arch/x86/xen/xen-asm_32.S
>> +++ b/arch/x86/xen/xen-asm_32.S
>> @@ -88,7 +88,11 @@ ENTRY(xen_iret)
>>  	 * avoid having to reload %fs
>>  	 */
>>  #ifdef CONFIG_SMP
>> +	pushw %fs
>> +	movl $(__KERNEL_PERCPU), %eax
>> +	movl %eax, %fs
>>  	GET_THREAD_INFO(%eax)
>> +	popw %fs
> 
> I don't think it's guaranteed that this can't fault.

If loading %fs faults when it is restored previously, the fixup zeros
the value.  However, this later load could still fault even if the first
succeeded.

Suggest copying the fixup section from the RESTORE_REGS macros in
arch/x86/kernel/entry_32.S

David
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