Re: [PATCH v2] xen/arm: Add 0x prefix when printing memory size in construct_domU

[Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>]

On 05/01/2023 11:19 am, Julien Grall wrote:
> On 05/01/2023 09:59, Ayan Kumar Halder wrote:
>> Hi Julien,
>
> Hi,
>
>> I have a clarification.
>>
>> On 05/01/2023 09:26, Julien Grall wrote:
>>> CAUTION: This message has originated from an External Source. Please
>>> use proper judgment and caution when opening attachments, clicking
>>> links, or responding to this email.
>>>
>>>
>>> Hi Stefano,
>>>
>>> On 04/01/2023 23:47, Stefano Stabellini wrote:
>>>> On Tue, 3 Jan 2023, Michal Orzel wrote:
>>>>> Printing memory size in hex without 0x prefix can be misleading, so
>>>>> add it. Also, take the opportunity to adhere to 80 chars line length
>>>>> limit by moving the printk arguments to the next line.
>>>>>
>>>>> Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
>>>>> ---
>>>>> Changes in v2:
>>>>>   - was: "Print memory size in decimal in construct_domU"
>>>>>   - stick to hex but add a 0x prefix
>>>>>   - adhere to 80 chars line length limit
>>>>
>>>> Honestly I prefer decimal but also hex is fine.
>>>
>>> decimal is perfect for very small values, but as we print the amount in
>>> KB it will become a big mess. Here some examples (decimal first, then
>>> hexadecimal):
>>>
>>>   512MB: 524288 vs 0x80000
>>>   555MB: 568320 vs 0x8ac00
>>>   1GB: 1048576 vs 0x100000
>>>   512GB: 536870912 vs 0x20000000
>>>   1TB: 1073741824 vs 0x40000000
>>>
>>> For power of two values, you might be able to find your way with
>>> decimal. It is more difficult for non power of two unless you have a
>>> calculator in hand.
>>>
>>> The other option I suggested in v1 is to print the amount in KB/GB/MB.
>>> Would that be better?
>>>
>>> That said, to be honest, I am not entirely sure why we are actually
>>> printing in KB. It would seems strange that someone would create a
>>> guest
>>> with memory not aligned to 1MB.
>>
>> For RTOS (Zephyr and FreeRTOS), it should be possible for guests to
>> have memory less than 1 MB, isn't it ?
>
> Yes. So does XTF. But most of the users are likely going allocate at
> least 1MB (or even 2MB to reduce the TLB pressure).
>
> So it would be better to print the value in a way that is more
> meaningful for the majority of the users.
>
>>> So I would consider to check the size is 1MB-aligned and then print the
>
> I will retract my suggestion to check the size. There are technically
> no restriction to run a guest with a size not aligned to 1MB.
> Although, it would still seem strange.

I have a need to extend tools/tests/tsx with a VM that is a single 4k
page.  Something which can execute CPUID in the context of a VM and
cross-check the results with what the "toolstack" (test) tried to configure.

Xen is buggy if it cannot operate a VM which looks like that, and a
bonus of explicitly testing like this is that it helps to remove
inappropriate checks.

~Andrew