Re: identify a Xen PV domU to fix devmem_is_allowed

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 03/21/2016 05:29 PM, Boris Ostrovsky wrote:
> On 03/15/2016 12:57 PM, Olaf Hering wrote:
>> On Tue, Mar 01, Boris Ostrovsky wrote:
>>
>>> on domU:
>>>
>>> [root@dhcp-burlington7-2nd-B-east-10-152-55-140 ~]# od -N 1 -j 4096 
>>> /dev/mem
>>> od: /dev/mem: read error: Bad address
>>> 0010000
>>> [root@dhcp-burlington7-2nd-B-east-10-152-55-140 ~]#
>>>
>>> with
>>>
>>> (XEN) mm.c:1767:d14v0 Bad L1 flags 10
>> How should we proceed with this bug?
>>
>
> I can't see any way to avoid calling xen_pv_domain() so what you 
> suggested should work. The only problem is that this will now cause 
> reserved areas to also return 0:
>
> # head /proc/iomem
> 00000000-00000fff : reserved <====
> 00001000-0009ffff : System RAM
> 000a0000-000fffff : reserved       <=====
>   000f0000-000fffff : System ROM
> 00100000-7fffffff : System RAM
>   01000000-0172a065 : Kernel code
>   0172a066-01d32b3f : Kernel data
>   01ec5000-02026fff : Kernel bss
> fee00000-fee00fff : Local APIC
>
> which I don't think they really should.
>
> How about this:
>
>     if (pagenr < 256 && !xen_pv_domain())
>         return 1;
>     if (iomem_is_exclusive(pagenr << PAGE_SHIFT))
>         return 0;
>     if (!page_is_ram(pagenr))
>         return 1;
>
>
> Also, while looking into this I noticed that pat_x_mtrr_type() will 
> make us switch from _PAGE_CACHE_MODE_WB to _PAGE_CACHE_MODE_UC_MINUS 
> when trying to mmap and this is what causes the hypervisor error 
> message and the splat in Linux. We make this switch despite the fact 
> that MTRR is disabled and therefore mtrr_type_lookup() returns 
> MTRR_TYPE_INVALID.
>
> Should we return req_type is MTRR is disabled?

Olaf,


Can you apply PAT changes from 
http://lists.xenproject.org/archives/html/xen-devel/2016-03/msg02127.html and 
see if it helps?

It should at least get rid of the splat (patch 3 is the one addresses 
no-MTRR problem that I mentioned above). We should still fix 
devmem_is_allowed() though.

-boris


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