From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Mon, 28 Aug 2000 13:56:34 +0100 (BST) From: Tigran Aivazian Subject: Re: How does the kernel map physical to virtual addresses? In-Reply-To: <20000825233748Z130198-15329+2857@vger.kernel.org> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org Return-Path: To: Timur Tabi Cc: Linux MM mailing list , Linux Kernel Mailing list List-ID: Hi, it is interesting to observe that many questions that deal with _details_ are answered quickly but questions related to fundamental concepts related to how Linux is designed, baffle all of us (since 0 people answered). So, is there really nobody in the whole world who can answer this? I would like to know the answer (about global kernel memory layout - i.e. what goes into PSE pages and what goes into normal ones, and how does PAE mode change the picture?) myself... I suppose one could find the answer by looking at each element of mem_map[] but it is always more comfortable to look for answers when one already knows the correct answer beforehand. Regards, Tigran On Fri, 25 Aug 2000, Timur Tabi wrote: > When my driver wants to map virtual to physical (and vice versa) addresses, it > calls virt_to_phys and phys_to_virt. All these macros do is add or subtract a > constant (PAGE_OFFSET) to one address to get the other address. > > How does the kernel configure the CPU (x86) to use this mapping? I was under > the impression that the kernel creates a series of 4MB pages, using the x86's > 4MB page feature. For example, in a 64MB machine, there would be 16 PTEs (PGDs? > PMDs?), each one mapping a consecutive 4MB block of physical memory. Is this > correct? Somehow I believe that this is overly simplistic. > > The reason I ask is that I'm confused as to what happens when a user process or > tries to allocate memory. I assume that the VM uses 4KB pages for this > allocatation. So do we end up with two virtual addresses pointing the same > physical memory? > > What happens if I use ioremap_nocache() on normal memory? Is that memory > cached or uncached? If I use the pointer obtained via phys_to_virt(), the > memory is cached. But if I use the pointer returned from ioremap_nocache(), the > memory is uncached. My understanding of x86 is that caching is based on > physical, not virtual addresses. If so, it's not possible for a physical > address to be both cached and uncached at the same. > > Could someone please straighten me out? > > > > -- > Timur Tabi - ttabi@interactivesi.com > Interactive Silicon - http://www.interactivesi.com > > When replying to a mailing-list message, please don't cc: me, because then I'll just get two copies of the same message. > - > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > Please read the FAQ at http://www.tux.org/lkml/ > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux.eu.org/Linux-MM/