From: nicolas.pitre@linaro.org (Nicolas Pitre)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH 6/8] ARM: mm: LPAE: Correct virt_to_phys patching for 64 bit physical addresses
Date: Tue, 23 Jul 2013 21:10:18 -0400 (EDT) [thread overview]
Message-ID: <alpine.LFD.2.03.1307231850350.15022@syhkavp.arg> (raw)
In-Reply-To: <1371858502-10083-7-git-send-email-santosh.shilimkar@ti.com>
On Fri, 21 Jun 2013, Santosh Shilimkar wrote:
> From: Sricharan R <r.sricharan@ti.com>
>
> The current phys_to_virt patching mechanism does not work
> for 64 bit physical addressesp. Note that constant used in add/sub
> instructions is encoded in to the last 8 bits of the opcode. So shift
> the _pv_offset constant by 24 to get it in to the correct place.
>
> The v2p patching mechanism patches the higher 32bits of physical
> address with a constant. While this is correct, in those platforms
> where the lowmem addressable physical memory spawns across 4GB boundary,
> a carry bit can be produced as a result of addition of lower 32bits.
> This has to be taken in to account and added in to the upper. The patched
> __pv_offset and va are added in lower 32bits, where __pv_offset can be
> in two's complement form when PA_START < VA_START and that can result
> in a false carry bit.
>
> e.g PA = 0x80000000 VA = 0xC0000000
> __pv_offset = PA - VA = 0xC0000000 (2's complement)
>
> So adding __pv_offset + VA should never result in a true overflow. So in
> order to differentiate between a true carry, a extra flag __pv_sign_flag
> is introduced.
I'm still wondering if this is worth bothering about.
If PA = 0x80000000 and VA = 0xC0000000 there will never be a real carry
to propagate to the high word of the physical address as the VA space
cannot be larger than 0x40000000.
So is there really a case where:
1) physical memory is crossing the 4GB mark, and ...
2) physical memory start address is higher than virtual memory start
address needing a carry due to the 32-bit add overflow?
It is easy to create (2) just by having a different user:kernel address
space split. However I wonder if (1) is likely. Sure you need a memory
alias in physical space to be able to boot, however you shouldn't need
to address that memory alias via virtual addresses for any
significant amount of time. In fact, as soon as the MMU is turned on,
there shouldn't be any issue simply using the final physical memory
addresses right away.
What am I missing?
Nicolas
next prev parent reply other threads:[~2013-07-24 1:10 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-06-21 23:48 [PATCH 0/8] ARM: mm: Extend the runtime patch stub for PAE systems Santosh Shilimkar
2013-06-21 23:48 ` [PATCH 1/8] ARM: mm: LPAE: use phys_addr_t appropriately in p2v and v2p conversions Santosh Shilimkar
2013-07-22 15:03 ` Nicolas Pitre
2013-06-21 23:48 ` [PATCH 2/8] ARM: mm: Introduce virt_to_idmap() with an arch hook Santosh Shilimkar
2013-06-21 23:48 ` [PATCH 3/8] ARM: mm: Move the idmap print to appropriate place in the code Santosh Shilimkar
2013-06-21 23:48 ` [PATCH 4/8] ARM: mm: Pass the constant as an argument to fixup_pv_table() Santosh Shilimkar
2013-06-21 23:48 ` [PATCH 5/8] ARM: mm: Add __pv_stub_mov to patch MOV instruction Santosh Shilimkar
2013-06-21 23:48 ` [PATCH 6/8] ARM: mm: LPAE: Correct virt_to_phys patching for 64 bit physical addresses Santosh Shilimkar
2013-07-24 1:10 ` Nicolas Pitre [this message]
2013-07-24 2:01 ` Santosh Shilimkar
2013-07-24 2:49 ` Nicolas Pitre
2013-07-24 11:50 ` Sricharan R
2013-07-24 12:07 ` Sricharan R
2013-07-24 14:04 ` Santosh Shilimkar
2013-07-24 20:21 ` Nicolas Pitre
2013-07-25 3:49 ` Sricharan R
2013-07-25 18:53 ` Santosh Shilimkar
2013-06-21 23:48 ` [PATCH 7/8] ARM: mm: Recreate kernel mappings in early_paging_init() Santosh Shilimkar
2013-06-21 23:48 ` [PATCH 8/8] ARM: keystone: Switch over to high physical address range Santosh Shilimkar
2013-06-22 1:51 ` [PATCH 0/8] ARM: mm: Extend the runtime patch stub for PAE systems Nicolas Pitre
2013-06-22 2:17 ` Santosh Shilimkar
2013-07-16 18:42 ` Santosh Shilimkar
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=alpine.LFD.2.03.1307231850350.15022@syhkavp.arg \
--to=nicolas.pitre@linaro.org \
--cc=linux-arm-kernel@lists.infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.