Re: [PATCH] reserve RAM below PHYSICAL_START

On Wed, 27 Feb 2008 01:33:25 +0100 Andrea Arcangeli wrote:

Hello,

this patch allows to prevent linux from using the ram below
PHYSICAL_START.

The "reserved RAM" can be mapped by virtualization software with to
create a 1:1 mapping between guest physical (bus) address and host
physical (bus) address. This will allow pci passthrough with DMA for
the guest with current production hardware that misses VT-d. The only
detail to take care of is the ram marked "reserved RAM failed". The
virtualization software must create for the guest an e820 map that
only includes the "reserved RAM" regions but if the guest touches
memory with guest physical address in the "reserved RAM failed" ranges
(linux guest will do that even if the ram isn't present in the e820
map), it should provide that as ram and map it with a not-ident
mapping. This should allow any linux kernel to run fine with pci
passthrough and hopefully any other OS too with all VT enabled
hardware.


Let me know if this can be merged, thanks!


Signed-off-by: Andrea Arcangeli <andrea@xxxxxxxxxxxx>

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1109,8 +1109,36 @@ config CRASH_DUMP
(CONFIG_RELOCATABLE=y).
For more details see Documentation/kdump/kdump.txt

+config RESERVE_PHYSICAL_START
+ bool "Reserve all RAM below PHYSICAL_START (EXPERIMENTAL)"
+ depends on !RELOCATABLE && X86_64
+ help
+ This makes the kernel use only RAM above __PHYSICAL_START.
+ All memory below __PHYSICAL_START will be left unused and
+ marked as "reserved RAM" in /proc/iomem. The few special
+ pages that can't be relocated at addresses above
+ __PHYSICAL_START and that can't be guaranteed to be unused
+ by the running kernel, will be marked "reserved RAM failed"

No comma.

+ in /proc/iomem. Those may or may be not used by the kernel
+ (for example smp trampoline pages would only be used if

SMP

+ CPU hotplug is enabled).
+
+ The "reserved RAM" can be mapped by virtualization software

Indent above with tab + 2 spaces, please.

+ with /dev/mem to create a 1:1 mapping between guest physical
+ (bus) address and host physical (bus) address. This will
+ allow pci passthrough with DMA for the guest using the ram

PCI RAM

+ with the 1:1 mapping. The only detail to take care of is the
+ ram marked "reserved RAM failed". The virtualization

RAM

+ software must create for the guest an e820 map that only
+ includes the "reserved RAM" regions but if the guest touches
+ memory with guest physical address in the "reserved RAM
+ failed" ranges (linux guest will do that even if the ram

Linux RAM

+ isn't present in the e820 map), it should provide that as
+ ram and map it with a non linear mapping. This should allow

RAM non-linear

+ any linux kernel to run fine and hopefully any other OS too.

Linux

+
config PHYSICAL_START
- hex "Physical address where the kernel is loaded" if (EMBEDDED || CRASH_DUMP)
+ hex "Physical address where the kernel is loaded" if (EMBEDDED || CRASH_DUMP || RESERVE_PHYSICAL_START)
default "0x1000000" if X86_NUMAQ
default "0x200000" if X86_64
default "0x100000"


---
~Randy
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