Re: [Xen-devel] [PATCH] libxc/arm: Correctly handle the difference between virtual and physical address

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-12-10 at 17:36 +0000, Julien Grall wrote:
> xc_dom_alloc_page deals with virtual address not physical address. When
> an ELF is loaded, virtual address and physical address may be different.

Can you give an example of the program headers of an ELF file (readelf
-l) which causes this? How was it constructed?

When we are building the guest the MMU is disabled so virt == phys.

If the ELF has virt != phys then how do we know whether it expects to be
launched with the MMU on or off? We can only really launch with the MMU
off, so do we require that the initial ELF entry point be PIC and know
how to enable the MMU?

Ian.

> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
> ---
>  tools/libxc/xc_dom_arm.c |   16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/tools/libxc/xc_dom_arm.c b/tools/libxc/xc_dom_arm.c
> index a40e04d..75a6f1c 100644
> --- a/tools/libxc/xc_dom_arm.c
> +++ b/tools/libxc/xc_dom_arm.c
> @@ -108,13 +108,15 @@ static int shared_info_arm(struct xc_dom_image *dom, 
> void *ptr)
>  static int vcpu_arm32(struct xc_dom_image *dom, void *ptr)
>  {
>      vcpu_guest_context_t *ctxt = ptr;
> +    uint32_t rambase = dom->rambase_pfn << XC_PAGE_SHIFT;
> +    uint32_t offset = dom->parms.virt_base - rambase;
>  
>      DOMPRINTF_CALLED(dom->xch);
>  
>      /* clear everything */
>      memset(ctxt, 0, sizeof(*ctxt));
>  
> -    ctxt->user_regs.pc32 = dom->parms.virt_entry;
> +    ctxt->user_regs.pc32 = dom->parms.virt_entry - offset;
>  
>      /* Linux boot protocol. See linux.Documentation/arm/Booting. */
>      ctxt->user_regs.r0_usr = 0; /* SBZ */
> @@ -125,7 +127,7 @@ static int vcpu_arm32(struct xc_dom_image *dom, void *ptr)
>       * like a valid pointer to a set of ATAGS or a DTB.
>       */
>      ctxt->user_regs.r2_usr = dom->devicetree_blob ?
> -        dom->devicetree_seg.vstart : 0xffffffff;
> +        (dom->devicetree_seg.vstart - offset) : 0xffffffff;
>  
>      ctxt->sctlr = SCTLR_GUEST_INIT;
>  
> @@ -280,15 +282,15 @@ int arch_setup_meminit(struct xc_dom_image *dom)
>  
>      if ( dom->devicetree_blob )
>      {
> -        const uint64_t rambase = dom->rambase_pfn << XC_PAGE_SHIFT;
> -        const uint64_t ramend = rambase + ( dom->total_pages << 
> XC_PAGE_SHIFT );
> +        const uint64_t virtbase = dom->parms.virt_base;
> +        const uint64_t virtend = virtbase + ( dom->total_pages << 
> XC_PAGE_SHIFT );
>          const uint64_t dtbsize = ROUNDUP(dom->devicetree_size, 
> XC_PAGE_SHIFT);
>  
>          /* Place at 128MB if there is sufficient RAM */
> -        if ( ramend >= rambase + 128*1024*1024 + dtbsize )
> -            dom->devicetree_seg.vstart = rambase + 128*1024*1024;
> +        if ( virtend >= virtbase + 128*1024*1024 + dtbsize )
> +            dom->devicetree_seg.vstart = virtbase + 128*1024*1024;
>          else /* otherwise at top of RAM */
> -            dom->devicetree_seg.vstart = ramend - dtbsize;
> +            dom->devicetree_seg.vstart = virtend - dtbsize;
>  
>          dom->devicetree_seg.vend =
>              dom->devicetree_seg.vstart + dom->devicetree_size;



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel