virtual cache, TLB, and OS

virtual cache, TLB, and OS

[xu feng]

Hi, 

I wish to be personally CC'ed the answers/comments
posted to the list in response to this post. 


I am confused about the following point regarding
Virtual Indexed Tag Indexed Cache and i would
appreciate any help

I have read in this article 
http://www.linuxjournal.com/article/7105 , the
following:
<< In virtually indexed, virtually tagged (VIVT)
caches...suffer from several other problems:
1- Virtual address translations usually are changed as
part of normal kernel operation, so the cache must pay
careful attention to changes in TLB entries (and
changes in address space) and flush cache lines whose
translations have changed.
2- Even in a single address space, multiple virtual
addresses may exist for the same physical address.
Each of these virtual addresses would be cached
separately, even though they represent the same data.
This is called the cache-line aliasing problem.
>>

I am just confused about the author:
1- first point, why the cache has to be bothered by
the change in the address logical-physical mapping
since it is a virtual cache??

2- could you please give me a situation where two
virtual addresses from the same process are mapped to
the same physical address? 

i can't see this happening since each process page is
allocated a dedicated frame. 

sorry if my questions seem obvious
thank you for your help










 
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Re: virtual cache, TLB, and OS

[Jeremy Fitzhardinge]

xu feng wrote:
> I am just confused about the author:
> 1- first point, why the cache has to be bothered by
> the change in the address logical-physical mapping
> since it is a virtual cache??
>   

If you:

   1. mmap file A at virtual address X
   2. use memory at X
   3. mmap file B at X
   4. look at X

If its a virtual cache, and it wasn't flushed at step 3, then step 4
will see A's contents rather than B's.

> 2- could you please give me a situation where two
> virtual addresses from the same process are mapped to
> the same physical address? 
>
> i can't see this happening since each process page is
> allocated a dedicated frame. 
>   

If you mmap a file multiple times, then the kernel will assign multiple
different virtual addresses for the same page cache page(s).


    J