vmalloc cached space..

vmalloc cached space..

[Ashish anand]

Hello,

As vmalloc returns KSEG2 cached memory , 
1.can I use a volatile pointer to treat it as safe as uncached..?
or
2. i should use vmalloc_prot (size, PAGE_KERNEL_UNCACHED) in umap.c.

Best regards,
Ashish

 

Re: vmalloc cached space..

[Ralf Baechle]

On Wed, Apr 16, 2003 at 03:10:17PM +0530, Ashish anand wrote:

> As vmalloc returns KSEG2 cached memory , 
> 1.can I use a volatile pointer to treat it as safe as uncached..?
> or

Volatile does not mean uncached.  Volatile means the C compiler should not
perform certain optimizations and is usually used for hardware accesses
that have side effects or variables that can be changed from some kind
of interrupt or signal handled and are also read by the main program.

> 2. i should use vmalloc_prot (size, PAGE_KERNEL_UNCACHED) in umap.c.

Uncached memory access is rarely necessary and so _extremly_ slow that it
should be avoided at almost any price.  Maybe you should describe what
you're trying to achieve, then let's see if there's a better suggestion
we can make.

  Ralf

Re: vmalloc cached space..

[Maciej W. Rozycki]

On Wed, 16 Apr 2003, Ralf Baechle wrote:

> > 2. i should use vmalloc_prot (size, PAGE_KERNEL_UNCACHED) in umap.c.
> 
> Uncached memory access is rarely necessary and so _extremly_ slow that it
> should be avoided at almost any price.  Maybe you should describe what
> you're trying to achieve, then let's see if there's a better suggestion
> we can make.

 In particular using cached accesses for stores and then doing writebacks
(plus invalidates if necessary) is still faster with WB caches. 

-- 
+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+--------------------------------------------------------------+
+        e-mail: macro@ds2.pg.gda.pl, PGP key available        +