Re: misunderstanding of the virtual memory

[Benjamin Teissier <ben.teissier@gmail.com>]

2013/5/9, Seth Jennings <sjenning@linux.vnet.ibm.com>:
> On Thu, May 09, 2013 at 10:33:21AM -0400, Ben Teissier wrote:
>>
>> Hi,
>>
>> I'm Benjamin and I'm studying the kernel. I write you this email
>> because I've a trouble with the mmu and the virtual memory. I try to
>> understand how a program (user land) can write something into the stack
>> (push ebp, for example), indeed, the program works with virtual address
>> (between 0x00000 and 0x8... if my memory is good) but at the hardware
>> side the address is not the same (that's why mmu was created, if I'm
>> right).
>
> Yes, this is the purpose of pages tables; to map virtual addresses to real
> memory addresses (more precisely virtual memory _pages_ to real memory
> pages).
>
>>
>> My problem is the following : how the data is wrote on the physical
>> memory. When I try a strace (kernel 2.6.32 on a simple program) I have
>> no hint on the transfer of data. Moreover, according to the wikipedia
>> web page on syscall (
>> https://en.wikipedia.org/wiki/System_call#The_library_as_an_intermediary
>> ), a call is not managed by the kernel. So, how the transfer between
>> virtual memory and physical memory is possible ?
>
> That is because writing to a memory location in userspace isn't an
> operation
> that requires a syscall or any kind of kernel intervention at all.  It is
> an
> assembly store instruction executed directly on the CPU by the program.
> The
> only time the kernel is involved in a store operation is if the virtual
> address
> translation doesn't exist in the TLB (or is write-protected, etc..), in
> which
> case the hardware generates a fault so the kernel take the required action
> to
> populate the TLB with the translation.
>
> Hope this answers your question.
>
> Seth
>
>

Hi,

Your answer is perfect, thanks a lot for your help !

Benjamin.

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