Re: [PATCH] reserve RAM below PHYSICAL_START

[Randy Dunlap]

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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