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Subject: Re: [PATCH 2/2] xen/pv: Call get_cpu_address_sizes to set
 x86_virt/phys_bits
From:   "M. Vefa Bicakci" <m.v.b@runbox.com>
To:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        linux-kernel@vger.kernel.org
Cc:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Juergen Gross <jgross@suse.com>,
        xen-devel@lists.xenproject.org, x86@kernel.org,
        stable@vger.kernel.org
References: <20180721194909.23903-1-m.v.b@runbox.com>
 <20180721194909.23903-2-m.v.b@runbox.com>
 <90bc371d-a296-51b3-8b78-62106c9690de@oracle.com>
 <d85c93da-4549-a7c1-af48-9c62cba8ae71@runbox.com>
Message-ID: <13b774d7-bc8c-039a-fbc0-52318f535883@runbox.com>
Date:   Sun, 22 Jul 2018 11:57:39 -0400
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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