Re: [Xen-devel] [v6][PATCH 2/2] xen:vtd: missing RMRR mapping while share EPT

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.09.14 at 09:40, <tiejun.chen@xxxxxxxxx> wrote:
> On 2014/9/19 15:10, Jan Beulich wrote:
>>>>> On 19.09.14 at 08:50, <tiejun.chen@xxxxxxxxx> wrote:
>>> On 2014/9/19 14:26, Jan Beulich wrote:
>>>>>>> On 19.09.14 at 03:20, <tiejun.chen@xxxxxxxxx> wrote:
>>>>> (XEN) [VT-D]dmar.c:807: found ACPI_DMAR_RMRR:
>>>>> (XEN) [VT-D]dmar.c:383:  endpoint: 0000:00:14.0
>>>>> (XEN) [VT-D]dmar.c:676:   RMRR region: base_addr ab805000 end_address
>>>>> ab818fff
>>>>> (XEN) [VT-D]dmar.c:807: found ACPI_DMAR_RMRR:
>>>>> (XEN) [VT-D]dmar.c:383:  endpoint: 0000:00:02.0
>>>>> (XEN) [VT-D]dmar.c:676:   RMRR region: base_addr ad000000 end_address
>>>>> af7fffff
>>>>
>>>> So how does passing through either of these work for a guest with
>>>> 4Gb or more of memory assigned with just the original 2 patches
>>>> (and with shared page tables)? There ought to be a collision detected
>>>> when trying to do the identity mapping.
>>>
>>> Do you mean this point, mfn_valid(mfn)? If yes, I remember we made
>>> agreement previously about how to cover three cases including this scenario:
>>>
>>> "
>>> #1: !mfn_valid(mfn)
>>>
>>> We can create those mapping safely.
>>>
>>> #2: mfn_x(mfn) == gfn && p2mt == p2m_mmio_direct && a == p2m_access_rw
>>>
>>> We already have these matched mappings.
>>>
>>> #3: Others
>>>
>>> Return with that waring message: "Cannot identity map d%d:%lx, already
>>> mapped to %lx but mismatch.\n"
>>> "
>>> And this is just as we did in patch #1:
>>>
>>> +
>>> +    if ( !mfn_valid(mfn) )
>>> +        ret = p2m_set_entry(p2m, gfn, _mfn(gfn), PAGE_ORDER_4K,
>>> p2m_mmio_direct,
>>> +                            p2m_access_rw);
>>> +    else if ( mfn_x(mfn) == gfn &&
>>> +              p2mt == p2m_mmio_direct &&
>>> +              a == p2m_access_rw )
>>> +        ret = 0;
>>> +    else
>>> +        printk(XENLOG_G_WARNING
>>> +               "Cannot identity map d%d:%lx, already mapped to %lx.\n",
>>> +               d->domain_id, gfn, mfn_x(mfn));
>>
>> Right, but the important point is that when the warning gets printed
>> -EBUSY gets returned, i.e. in the end the device assignment is to fail.
>> Are you seeing the warning when creating a large enough guest? If
> 
> My platform just own 4G memory, so after I try to set 'memory=15360' in 
> domu.cfg, I can't boot such a VM:
> 
> # xl cr domu.cfg
> Parsing config from domu.cfg
> libxl: error: libxl.c:4202:libxl_set_memory_target: new target 0 for 
> dom0 is below the minimum threshhold
> ...
> failed to free memory for the domain
> 
>> not - can you explain why you don't see it (as I can't)?
> 
> Do you know exactly how to test this case as you expect here? Then I can 
> take a further look to step on your question. Or I guess you are hinting 
> something wrong should be happened but I never realize that previously.

It should suffice to give 3 Gb (or event slightly less) of memory to
the DomU (if your Dom0 can hopefully tolerate running with just 1Gb).

Jan


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