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

["M. Vefa Bicakci" <m.v.b@xxxxxxxxxx>]

On 07/23/2018 11:04 AM, Boris Ostrovsky wrote:
> On 07/22/2018 11:57 AM, M. Vefa Bicakci wrote:
> > 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.
>
> Ah, that's why it hasn't been detected.
>
>
> > 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.
>
> No, I think your patch is good. The only thing I'd suggest is to move
> the call a few lines down. The way it is placed now may create
> impression that we are calling get_cpu_address_sizes() to figure out NX
> support.
Sorry for the delay, and thank you for your comments! I will
send an updated version of this patch in a few moments.

Vefa

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