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

[Boris Ostrovsky]

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.


-boris