linux-kernel - Re: [PATCH 2/2] xen/pv: Call get_cpu_address_sizes to set x86_virt/phys

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Date:   Tue, 24 Jul 2018 08:42:28 -0400
From:   "M. Vefa Bicakci" <m.v.b@...box.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        linux-kernel@...r.kernel.org
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Juergen Gross <jgross@...e.com>,
        xen-devel@...ts.xenproject.org, x86@...nel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH 2/2] xen/pv: Call get_cpu_address_sizes to set
 x86_virt/phys_bits

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