Re: [PATCH] reserve RAM below PHYSICAL_START

Randy Dunlap
2008-02-27T23:52:19Z

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@qumranet.com>
>
> 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