Re: [Xen-devel] Xen on Seattle

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

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.

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