Re: [Xen-devel] Xen on Seattle

[Suravee Suthikulanit <suravee.suthikulpanit@xxxxxxx>]

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


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