linux-kernel - Re: [Xen-devel] [PATCH 1/4] xen/PVH: Replace GDT

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Date:   Mon, 30 Apr 2018 14:07:43 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Roger Pau Monné <roger.pau@...rix.com>
Cc:     linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        jgross@...e.com, stable@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH 1/4] xen/PVH: Replace GDT_ENTRY with explicit
 constant

On 04/30/2018 12:57 PM, Roger Pau Monné wrote:
> On Mon, Apr 30, 2018 at 12:23:36PM -0400, Boris Ostrovsky wrote:
>> Latest binutils release (2.29.1) will no longer allow proper computation
>> of GDT entries on 32-bits, with warning:
>>
>> arch/x86/xen/xen-pvh.S: Assembler messages:
>> arch/x86/xen/xen-pvh.S:150: Warning: shift count out of range (32 is not between 0 and 31)
>> arch/x86/xen/xen-pvh.S:150: Warning: shift count out of range (40 is not between 0 and 31)
>> arch/x86/xen/xen-pvh.S:150: Warning: shift count out of range (32 is not between 0 and 31)
>> arch/x86/xen/xen-pvh.S:152: Warning: shift count out of range (32 is not between 0 and 31)
>> arch/x86/xen/xen-pvh.S:152: Warning: shift count out of range (40 is not between 0 and 31)
>> arch/x86/xen/xen-pvh.S:152: Warning: shift count out of range (32 is not between 0 and 31)
>>
>> Use explicit value of the entry instead of using GDT_ENTRY() macro.
>>
>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
>> Cc: stable@...r.kernel.org
>> ---
>>  arch/x86/xen/xen-pvh.S | 6 +++---
>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/xen/xen-pvh.S b/arch/x86/xen/xen-pvh.S
>> index e1a5fbe..934f7d4 100644
>> --- a/arch/x86/xen/xen-pvh.S
>> +++ b/arch/x86/xen/xen-pvh.S
>> @@ -145,11 +145,11 @@ gdt_start:
>>  	.quad 0x0000000000000000            /* NULL descriptor */
>>  	.quad 0x0000000000000000            /* reserved */
>>  #ifdef CONFIG_X86_64
>> -	.quad GDT_ENTRY(0xa09a, 0, 0xfffff) /* __KERNEL_CS */
>> +	.quad 0x00af9a000000ffff            /* __BOOT_CS */
>>  #else
>> -	.quad GDT_ENTRY(0xc09a, 0, 0xfffff) /* __KERNEL_CS */
>> +	.quad 0x00cf9a000000ffff            /* __BOOT_CS */
> Maybe it would be cleaner to use something like:

I actually considered all of these and ended up with a raw number
because it seems to be a convention in kernel (and Xen too, apparently) 
to use raw values in .S files.

Kernel is using now GDT_ENTRY_INIT() which is a C macro. There is one
other location where GDT_INIT() is used (arch/x86/boot/pm.c) and,
incidentally, it also generates this warning IIRC.

I really don't want to move definition to C code just to use a macro ---
I don't think C code needs to be exposed to this GDT.


>
> .word 0xffff /* limit */
> .word 0      /* base */
> .byte 0      /* base */
> .byte 0x9a   /* access */
> #ifdef CONFIG_X86_64
> .byte 0xaf   /* flags plus limit */
> #else
> .byte 0xcf   /* flags plus limit */
> #endif
> .byte 0      /* base */


I, in fact, started with something like this. But if you repeat this 4
times you will probably see why I decided against it ;-)


-boris


>
> Or try to fix the GDT_ENTRY macro, maybe if you prepend extra 0's to
> make the values 64bit that would prevent the warnings?
>
> Or declare the GDT in enlighten_pvh in C and use it here?
>
> Also, IIRC the base/limit values are ignored in long mode.
>
> Thanks, Roger.