Re: [PATCH 2/2] xen/pv: Call get_cpu_address_sizes to set x86_virt/phys_bits

[M. Vefa Bicakci]

On 07/21/2018 07:17 PM, M. Vefa Bicakci wrote:
> On 07/21/2018 05:25 PM, Boris Ostrovsky wrote:
> > On 07/21/2018 03:49 PM, M. Vefa Bicakci wrote:
> > > diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> > > index 439a94bf89ad..87afb000142a 100644
> > > --- a/arch/x86/xen/enlighten_pv.c
> > > +++ b/arch/x86/xen/enlighten_pv.c
> > > @@ -1257,6 +1257,7 @@ asmlinkage __visible void __init xen_start_kernel(void)
> > >       /* Work out if we support NX */
> > >       get_cpu_cap(&boot_cpu_data);
> > > +    get_cpu_address_sizes(&boot_cpu_data);
> > >       x86_configure_nx();
> >
> >
> > Have you observed any problems without this call? get_cpu_cap() is only
> > called here to set X86_FEATURE_NX, and is then called again, together
> > with get_cpu_address_sizes(), from early_identify_cpu().
>
> Thank you for the reviews! Without the call to get_cpu_address_sizes,
> paravirtualized virtual machines do not boot up kernels with versions
> 4.17 and up at all; this includes dom0 and domU. No domU logs are
> generated in dom0's /var/log/xen/console/ directory either, despite
> having earlyprintk=xen on the kernel command line for my test domU.

Hello Boris,

I debugged this further with a debugging version of Xen (so that I can
get early kernel print-outs via the "xen_raw_console_write" function),
and I found the root cause of the boot up failure.

In summary, the issue is due to the following call path in version
4.17 (and higher, I assume), which the kernel goes through /only/ when
CONFIG_DEBUG_VIRTUAL is enabled:

enlighten_pv.c::xen_start_kernel
  mmu_pv.c::xen_reserve_special_pages
    page.h::__pa
      physaddr.c::__phys_addr
        physaddr.h::phys_addr_valid // uses boot_cpu_data.x86_phys_bits

The return value of phys_addr_valid is used with the VIRTUAL_BUG_ON macro,
which evaluates to BUG_ON in case CONFIG_DEBUG_VIRTUAL is enabled.

It looks like the call to get_cpu_address_size is required in the
xen_start_kernel function. Perhaps there is a more elegant way to
resolve this issue as well.

Another approach could be to check in the phys_addr_valid function whether
boot_cpu_data.x86_phys_bits has been initialized or not, I think, but I am
not sure about the correctness of this approach.

Thank you,

Vefa