Question about memory management in L4 Fiasco O.C + L4re

Mahdi Aichouch foxmehdi at gmail.com
Tue Jul 28 16:10:04 CEST 2015


Hello Adam,

I have a question about the way Fiasco.OC handles IPC.

If we take a simple example of one thread in address space A that sends an
array of characters to a second
thread located in an address space B.
Since such an array might be large and cannot be transferred using the
registers.

How it is transferred to the second thread address space?

Is it at first allocated on the first thread stack, then copied by Fiasco
kernel to
the second thread address space?

Thank you very much for your answer.

Best regards,
Mahdi


On Fri, Jul 24, 2015 at 9:12 AM, Mahdi Aichouch <foxmehdi at gmail.com> wrote:

> Hello Adam,
>
> What I mean by accessing memory partition is: does Fiasco.OC accesses
> memory
> reserved for L4Linux when handling system-call issued by L4Linux guest for
> example
> or any other operation.
>
> Thank you very much for your answer.
>
> Best regards,
> Mahdi
>
>
> On Thu, Jul 23, 2015 at 5:32 PM, Mahdi Aichouch <foxmehdi at gmail.com>
> wrote:
>
>> Hello Adam,
>>
>> I would like to know if Fiasco.OC kernel accesses memory partition
>> allocated to an L4Linux guest?
>> And for what reasons it has to do it?
>>
>> Thank you very much for your answer.
>>
>> Best regards,
>> Mahdi
>>
>>
>>
>>
>> On Wed, Jul 22, 2015 at 3:19 PM, Mahdi Aichouch <foxmehdi at gmail.com>
>> wrote:
>>
>>> Hello Adam,
>>>
>>> According to Fiasco O.C/L4re documentation Sigma0 is the root pager,
>>> that is, it is responsible of resolving page fault of user-level tasks,
>>> right?
>>>
>>> Knowing that L4Linux is created as a user-level task, thus any memory
>>> page fault occurring
>>> in L4Linux or its user processes logically has to be handled through
>>> Sigma0.
>>>
>>> However, as you previously mentioned in one of your answers, a handling
>>> of
>>> a page fault occurring in L4Linux does not involve any L4Re object
>>> including Sigma0.
>>>
>>> My question is: what configuration and / or operations have been made
>>> in order to force that a memory page fault in L4Linux has to be handled
>>> only by Fiasco and L4Linux and not going through L4Re objects?
>>> And where these operations are written in the source code.
>>>
>>> Thank you very much for your answer.
>>>
>>> Best regards.
>>> Mahdi
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> On Tue, Jul 21, 2015 at 4:06 PM, Mahdi Aichouch <foxmehdi at gmail.com>
>>> wrote:
>>>
>>>> Hello Adam,
>>>>
>>>> I would like to understand how L4 Fiasco manages the virtual memory of
>>>> a L4Linux guest.
>>>>
>>>> Does L4 Fiasco maintains a "shadow page table" in order to perform a
>>>> two-level memory address translation:
>>>> guest virtual memory --> guest real memory --> host physical memory?
>>>>
>>>> If the answer is yes, where the "shadow page table" is maintained?
>>>> which L4 Fiasco object
>>>> is responsible of this operation?
>>>>
>>>> If we suppose that an L4Linux guest has for example a 64MB of fixed
>>>> physical memory region,
>>>> in case a new L4Linux user task arrives and there is no available space
>>>> to allocate
>>>> a new page for this task, an already used page has to be unmapped,
>>>> right?
>>>>
>>>> If the answer is yes, then where the dirty page will be swapped?
>>>>
>>>> Thank you very much for your answer.
>>>>
>>>> Best regards,
>>>> Mahdi
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> On Mon, Jul 20, 2015 at 4:36 PM, Mahdi Aichouch <foxmehdi at gmail.com>
>>>> wrote:
>>>>
>>>>> Hello Adam,
>>>>>
>>>>> If I correctly understood, and If we suppose that an L4Linux instance
>>>>> is started for example with this command line options:
>>>>> ... earlyprintk=1 em=64M ...
>>>>>
>>>>> This means that a 64MB of physical memory region composed of a set of
>>>>> contiguous physical frames is allocated to L4Linux, right?
>>>>>
>>>>> L4Linux will use a part of this 64MB of physical memrory for its own
>>>>> kernel execution, and
>>>>> the rest of physical memory space will be used by L4Linux to load
>>>>> user-level programs
>>>>> and allocate physical memory for them, right?
>>>>>
>>>>> If a user-level program on top of L4Linux generates a page-fault.
>>>>> Are the L4 Fiasco kernel and L4re involved in the execution flow to
>>>>> handle the page-fault?
>>>>>
>>>>> Thank you very much for your clarification.
>>>>>
>>>>> Best regards,
>>>>>
>>>>> Mahdi
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On Fri, Jul 17, 2015 at 10:36 AM, Mahdi Aichouch <foxmehdi at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> Hello Adam,
>>>>>>
>>>>>> The memory regions are exclusive. Specifying exactly which physical
>>>>>>> memory an L4Linux is getting is currently not possible but I guess
>>>>>>> you're using the numbers just as an example?
>>>>>>
>>>>>>
>>>>>> Yes, I am giving these numbers as an example.
>>>>>> However, is it possible to know what are the start address and the end address
>>>>>> of a physical memory partition allocated to a L4Linux instance?
>>>>>>
>>>>>> The memory is completely mapped initially, so no page fault should
>>>>>>> happen. As probably nobody will take it away again it should also stay
>>>>>>> like this.
>>>>>>
>>>>>> Does this apply also to user-level programs executed on top of L4Linux.
>>>>>>
>>>>>> You can put showpfexc=1 on the cmdline to see any in-kernel page fault.
>>>>>>>
>>>>>> There shouldn't be any (except in the outside wrapper code as I see
>>>>>>> which can be changed by launching L4Linux with the eager_map flag).
>>>>>>
>>>>>> Could please give some explanation about what is the wrapper code? And Where to set the "eager_map flag" option?
>>>>>>
>>>>>> Thank you very much for your answer!
>>>>>>
>>>>>> Best regards,
>>>>>>
>>>>>> Mahdi
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Wed, Jul 15, 2015 at 11:53 AM, Mahdi Aichouch <foxmehdi at gmail.com>
>>>>>> wrote:
>>>>>>
>>>>>>> Hello,
>>>>>>>
>>>>>>> I have some questions concerning memory management in Fiasco O.C  +
>>>>>>> L4re.
>>>>>>>
>>>>>>> I would like to test two L4Linux instances, and for each L4Linux
>>>>>>> instance I want to reserve one static fixed physical memory
>>>>>>> partition.
>>>>>>>
>>>>>>> For instance, one 128MB for the first L4Linux and one 128MB for the
>>>>>>> second.
>>>>>>>
>>>>>>> Knowing that Fiasco O.C. uses "on demand paging" to allocate pages
>>>>>>> for
>>>>>>> user tasks, is it possible that each partition of L4Linux could be
>>>>>>> allocated
>>>>>>> a static physical memory region composed of contiguous physical
>>>>>>> frames.
>>>>>>>
>>>>>>> For instance, one memory region starting from 128MB to 256MB for the
>>>>>>> first L4Linux,
>>>>>>> and second memory region from 256MB to 512MB for the second L4Linux.
>>>>>>> Each memory region is allocated exclusively to one L4Linux instance.
>>>>>>>
>>>>>>> I would like to know if it is possible to load all the code + data
>>>>>>> of a L4Linux instance
>>>>>>> into its reserved memory partition, so no page fault will be
>>>>>>> encountered during runtime.
>>>>>>>
>>>>>>> Is it possible to tell me if these above operations could be
>>>>>>> realized in Fiasco O.C and L4re?
>>>>>>>
>>>>>>> If the answer is yes, is it possible to tell me what are the objects
>>>>>>> that should be used or
>>>>>>> adapted in order to implement these?
>>>>>>>
>>>>>>> What are the issues that I should pay attention to?
>>>>>>>
>>>>>>> Many thanks in advance for your answer.
>>>>>>>
>>>>>>> Best regards,
>>>>>>>
>>>>>>> Mahdi
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
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