Re: [Xen-devel] [PATCH] xen/arm64: fix section shift when mapping 2MB block in boot page table

[Chen Baozi <baozich@xxxxxxxxx>]

On Jan 14, 2014, at 17:57, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:

> On Tue, 2014-01-14 at 01:06 +0800, Chen Baozi wrote:
>> Section shift for level-2 page table should be #21 rather than #20.
> 
> Thanks, are you finding these issues on actual hardware or just by
> inspection?

Oh, I noticed this because I was inspecting my Mini-OS’s code by
comparing with this. :-)

> 
> I think this change is correct, also arm32 has:
> 
>        1:      /* Setup boot_second: */
>                ldr   r4, =boot_second
>                add   r4, r4, r10            /* r1 := paddr (boot_second) */
> 
>                lsr   r2, r9, #20            /* Base address for 2MB mapping */
>                lsl   r2, r2, #20
>                orr   r2, r2, #PT_UPPER(MEM) /* r2:r3 := section map */
>                orr   r2, r2, #PT_LOWER(MEM)
> 
> Either they are both wrong or both right (I think both wrong).
> 
> The effect of this error is that bit #20 of paddr(boot_second) is
> preserved as bit #20 of the PTE, rather than being cleared. bit #20 of
> an entry at this level is UNK/SBZP -- so we survive this mistake even if
> paddr(boot_second) happens to have bit #20 set.
> 
> Really this code should use {FIRST,SECOND,THIRD}_SHIFT, and this file
> already includes asm/page.h so they should be available…

Ok, I’ll write the V2 to fix them all.

Cheers,

Baozi

> 
>> 
>> Signed-off-by: Chen Baozi <baozich@xxxxxxxxx>
>> ---
>> xen/arch/arm/arm64/head.S | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
>> index bebddf0..ad35f60 100644
>> --- a/xen/arch/arm/arm64/head.S
>> +++ b/xen/arch/arm/arm64/head.S
>> @@ -292,8 +292,8 @@ skip_bss:
>>         ldr   x4, =boot_second
>>         add   x4, x4, x20            /* x4 := paddr (boot_second) */
>> 
>> -        lsr   x2, x19, #20           /* Base address for 2MB mapping */
>> -        lsl   x2, x2, #20
>> +        lsr   x2, x19, #21           /* Base address for 2MB mapping */
>> +        lsl   x2, x2, #21
>>         mov   x3, #PT_MEM            /* x2 := Section map */
>>         orr   x2, x2, x3
>> 
> 
> 


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