Re: Xen on Seattle

[Suravee Suthikulanit <suravee.suthikulpanit@amd.com>]

On 9/2/2014 1:15 PM, Stefano Stabellini wrote:
> On Tue, 2 Sep 2014, Suravee Suthikulanit wrote:
>> On 9/2/2014 1:06 PM, Stefano Stabellini wrote:
>>> On Tue, 2 Sep 2014, Suravee Suthikulanit wrote:
>>>> Hi all,
>>>>
>>>> I am running into an issue booting Xen on Seattle platform where dom0
>>>> kernel
>>>> failed to make the following hypercall in arch/arm/xen/enlighten.c:
>>>> xen_percpu_init().
>>>>
>>>>    HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_info, cpu, &info)
>>>>
>>>> After investigation, the hypercall failed in the xen/common/domain.c:
>>>> map_vcpu_info(), where it check
>>>>
>>>>       if ( offset > (PAGE_SIZE - sizeof(vcpu_info_t)) )
>>>>           return -EINVAL;
>>>>
>>>> Here:
>>>>
>>>> (XEN) DEBUG0: map_vcpu_info: offset = 0x6790
>>>> (XEN) DEBUG0: map_vcpu_info: PAGE_SIZE = 0x1000
>>>> (XEN) DEBUG0: map_vcpu_info: sizeof(vcpu_info_t) = 0x30
>>>>
>>>> If I understand correctly, it seems that the hypervisor is making
>>>> assumption
>>>> that the Dom0 guest is using 4K page size.  The PAGE_SHIFT in
>>>> include/asm-arm/config.h is currently set to 12.  So, I have tried
>>>> changing
>>>> this to 16 (for 64K page). However, there is a restriction on the 16-bit
>>>> value
>>>> of the struct active_grant_entry { length:16; }. So, I cannot boot dom0
>>>> with
>>>> 64K page size. Is this accurate?
>>>
>>> Hello Suravee,
>>> yes, that is correct: all pages used in hypercalls and shared with the
>>> hypervisor need to be 4K pages.
>>>
>>
>> Ok, the next question would be, would it make sense/possible to extend the
>> support to allow 64K pages?
>
> It might make sense, but it is not trivial: all Xen hypercall interfaces
> are built for 4K pages and would need a significant revamp.  Certainly
> too late for the next release. It might be something to look at for Xen
> 4.6.
>
> There is nothing preventing a guest kernel from using 64K pages anywhere
> else though.
>
The alternative is to try booting with dom0 kernel configured with 4K 
pages. Currently, with 3.16 kernel, this is not working yet with Seattle 
since we would need level 4 page table support to boot with 4K page size.

There is a patch from Catalin which should add support for level 4 page. 
  Let us experiment with that and get back with you guys.

Suravee