Re: [Xen-devel] Xen on Seattle

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 2 Sep 2014, Suravee Suthikulanit wrote:
> 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.

Yeah, we typically use 4K pages and 2M superpages in guest kernels.


> 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.

That would be great, thanks!

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