Re: [Xen-devel] [PATCH] xen/arm: Allocate memory for dom0 from the bottom with the 1:1 Workaround

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Tue, 2013-10-22 at 11:51 +0100, Julien Grall wrote:
> On Linux, the option CONFIG_ARM_PATCH_PHYS_VIRT (by default enabled) allows
> the Kernel to be loaded anywhere (or nearly) by patching the translation
> pv<->virt at boot time.
> 
> The current solution in Linux assuming that the delta physical address -
> virtual address is always negative. A positive delta will destroy all the
> optimisation to modify only a part of the translation instruction (add/sub).
> 
> By default, Xen is allocating memory from the top of memory and then
> goes down. To avoid booting issue with Linux, we must allocate memory
> from the bottom (ie starting from 0).
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>

Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>

OOI how low does the memory get allocated in practice? We deliberately
load Xen and place xenheap up high so I guess for dom0 we can't get
memory from pretty low down?

> ---
>  xen/arch/arm/domain_build.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
> index 0698f83..69ea6a0 100644
> --- a/xen/arch/arm/domain_build.c
> +++ b/xen/arch/arm/domain_build.c
> @@ -70,12 +70,19 @@ static int set_memory_reg_11(struct domain *d, struct 
> kernel_info *kinfo,
>      int reg_size = dt_cells_to_size(dt_n_addr_cells(np) + 
> dt_n_size_cells(np));
>      paddr_t start;
>      paddr_t size;
> -    struct page_info *pg;
> +    struct page_info *pg = NULL;
>      unsigned int order = get_order_from_bytes(dom0_mem);
>      int res;
>      paddr_t spfn;
> +    unsigned int bits;
> +
> +    for ( bits = PAGE_SHIFT + 1; bits < PADDR_BITS; bits++ )
> +    {
> +        pg = alloc_domheap_pages(d, order, MEMF_bits(bits));
> +        if ( pg != NULL )
> +            break;
> +    }
>  
> -    pg = alloc_domheap_pages(d, order, 0);
>      if ( !pg )
>          panic("Failed to allocate contiguous memory for dom0\n");
>  



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