On Thu, 3 Apr 2008, Rene Herman wrote:
> On 03-04-08 19:51, Robert P. J. Day wrote:
> > generally, the virtual addresses in the interval [3G, 3G+896M)
> > are mapped to physical addresses simply by subtracting the offset
> > of 3G, so these are additionally referred to as "logical"
> > addresses (in hindsight, not a terrific choice of name but it
> > seems to have caught on). that leaves the kernel address space of
> > [3G+896, 4G) -- HIGHMEM -- as the only way to refer to physical
> > memory above 1G and, obviously, for which extra address
> > translation needs to be done. so far, so good?
>
> As the only way to refer to physical memory above 896M, the part of
> memory called highmem. Highmem is not the [3G+896M,4G) range, that
> range is the vmalloc space and the window into which highmem pages
> are _mapped_.
quite right, i need to be more careful with my terminology.
> Also, no extra address translations need to be done on highmem pages
> over lowmem pages, they just need to be mapped after which address
> translation is the same as for any other mapped page.
yes, i worded that badly, thanks.
> > so, some questions?
> >
> > * if i absolutely *know* that i'm building a kernel for a system
> > that doesn't have more than 1G of RAM, do i need HIGHMEM at all?
>
> s/1G/896M/ and no.
hang on ... if i *know* i'm building a (32-bit) kernel for a system
with exactly 1G of RAM, are you saying i'll need HIGHMEM support?
why? if you read the "Help" in the kernel configuration, it describes
the option CONFIG_HIGHMEM4G with:
"Select this if you have a 32-bit processor and between 1 and 4
gigabytes of physical RAM."
it doesn't say between 896M and 4G. i would have thought that, if i
had no more than 1G of RAM, i could select no HIGHMEM support, at
which the entire 1G kernel address space would be logical. you're
saying that's not true?
rday
--
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry:
Have classroom, will lecture.
http://crashcourse.ca Waterloo, Ontario, CANADA
========================================================================
--
To unsubscribe from this list: send an email with
"unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx
Please read the FAQ at http://kernelnewbies.org/FAQ
