Re: [Xen-devel] [PATCH] xen/arm: p2m: configure pa_range_info table to support 42 bit PA systems.

[Feng Kan OS <fengkan@xxxxxxxxxxxxxxxxxxxxxx>]

Thanks Julien.

On 3/15/19 4:21 AM, Julien Grall wrote:
> Hi,
> 
> Thank you for posting the patch.
> 
> Title: No need for full stop in the commit title.
> 
> On 15/03/2019 08:34, Vishnu Pajjuri OS wrote:
>> Current pa_range_info table's configuration prevents 42 bit PA systems
>> from booting DOM0. This patch modifies t0sz=22 and root_order=3
>> to expose 42-bit IPA (Intermediate Physical Address).
>>
>> It is required for hardware which is having peripherals above 40 bits.
> 
> The commit message is a bit confusing, you first say Xen does not boot 
> on 42-bits PA systems but towards the end of the commit you restrict to 
> only platform that have address wired above 40-bits.
> 
> The limitation was introduced because existing platform back then had 
> all address wired below 40-bits. So we can limit to 40-bits and allocate 
> 2 pages rather than 8 pages for each page-tables.
> 
> While I am perfectly fine to see this for Dom0, I am still unsure this 
> is the right things for guest. Do you currently have any use case for 
> 42-bits (4TB) guest?
I agree there is no use case at the moment for 42 bit guest, it is a fix
for DOM0 booting with peripheral and memory above 40 bits of PA. The 
other option is to keep separate table for the guest?
> 
> I can't remember which other platforms support 42-bits PA. I think at 
> that time it was X-Gene. As long as no current embedded platform we 
> support use 42-bit PA, this change may be ok. Stefano do you recall what 
> was the platform?
Ampere eMAG platform is essentially the continuation of X-Gene. These 
systems are targeted as servers with upto 1TB of RAM.
> 
> In any case, the new behavior (and consequence) needs to be clearly 
> explained in the commit message.
Got it, we will resubmit if Stefano is okay with the change.

> 
>>
>> Signed-off-by: Vishnu <vishnu@xxxxxxxxxxxxxxxxxxxxxx>
>> Signed-off-by: Feng Kan <fengkan@xxxxxxxxxxxxxxxxxxxxxx>
>> ---
>>   xen/arch/arm/p2m.c | 5 ++++-
>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
>> index c38bd7e16e..c8e7598d81 100644
>> --- a/xen/arch/arm/p2m.c
>> +++ b/xen/arch/arm/p2m.c
>> @@ -1992,10 +1992,13 @@ void __init setup_virt_paging(void)
>>       } pa_range_info[] = {
>>           /* T0SZ minimum and SL0 maximum from ARM DDI 0487A.b Table 
>> D4-5 */
>>           /*      PA size, t0sz(min), root-order, sl0(max) */
>> +    /* TODO: 42 bit PA systems allocate 8 pages by default
>> +     * for both DOM0 and guests.
>> +     */
> 
> Xen uses soft-tab and not hard-tab. Also, multi-lines comment in Xen 
> looks like:
> 
> /*
>   * Foo
>   */
> 
> Lastly, what you have after the TODO is more a fact than a TODO. If we 
> decide to keep the TODO, then it should explain what we need to do.
> 
>>           [0] = { 32,      32/*32*/,  0,          1 },
>>           [1] = { 36,      28/*28*/,  0,          1 },
>>           [2] = { 40,      24/*24*/,  1,          1 },
>> -        [3] = { 42,      24/*22*/,  1,          1 },
>> +        [3] = { 42,      22/*22*/,  3,          1 },
>>           [4] = { 44,      20/*20*/,  0,          2 },
>>           [5] = { 48,      16/*16*/,  0,          2 },
>>           [6] = { 0 }, /* Invalid */
>> -- 
>> 2.19.1
>>
> 
> Cheers,
> 
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