Re: [Xen-users] Error accessing memory mapped by xenforeignmemory_map()

[Brett Stahlman <brettstahlman@xxxxxxxxx>]

Julien,

On Tue, Oct 31, 2017 at 3:44 PM, Julien Grall <julien.grall@xxxxxxx> wrote:
> Hi,
>
> On 31/10/2017 19:17, Brett Stahlman wrote:
>> Ok. So I'm thinking the relevant address would be 0xffffffc000080000,
>> which is where System.map places the kernel's "_text" segment. IIUC,
>> aarch64 Linux uses 39-bit addresses in a range controlled by TTBR1,
>> beginning at 0xffffff8000000000. I'm assuming the kernel's address
>> space uses a direct, 1-to-1 mapping (i.e., guest virtual == guest
>> physical). Correct? Also, should a signed or unsigned right shift be
>> used to convert a kernel address to a pfn? I.e., would the pfn
>> corresponding to 0xffffffc000080000 be 0xfffffffffc000080 or
>> 0x000ffffffc000080?
>
> The kernel does not direct map the memory (e.g guest virtual
> != guest physical). 0xffffffc000080000 would be virtual
> address of the start of the kernel.
>
> I don't think Linux is moving the kernel within the physical
> address space after boot. Assuming you don't use UEFI in
> the guest and you are using Xen 4.6 or later, the kernel
> will be loaded at the guest physical address 0x40008000.
>
> You can also find this information via xl create when it
> is in verbose mode (xl -vvv create):
>
> domainbuilder: detail: xc_dom_pfn_to_ptr_retcount: domU mapping: pfn 
> 0x40008+0x7b4 at 0xffff94696000
> domainbuilder: detail: xc_dom_alloc_segment:   kernel       : 0x40008000 -> 
> 0x407bc000  (pfn 0x40008 + 0x7b4 pages)
> domainbuilder: detail: xc_dom_load_zimage_kernel: called
> domainbuilder: detail: xc_dom_load_zimage_kernel: kernel seg 
> 0x40008000-0x407bc000

Bingo! Using this approach, I was finally able to map my guest kernel's
head page (at 0x40008000) and read the expected data. I appreciate your
patience...

Sincerely,
Brett S.

>
> Cheers,
>
> --
> Julien Grall

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