Re: [Xen-devel] [v7][RFC][PATCH 06/13] hvmloader/ram: check if guest memory is out of reserved device memory maps

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 30.10.14 at 04:11, <tiejun.chen@xxxxxxxxx> wrote:
> On 2014/10/29 17:15, Jan Beulich wrote:
>>>>> On 29.10.14 at 08:43, <tiejun.chen@xxxxxxxxx> wrote:
>>> In VT-D specification, I just see,
>>>
>>> "The RMRR regions are expected to be used for legacy usages (such as
>>> USB, UMA Graphics, etc.) requiring reserved memory. Platform designers
>>> should avoid or limit use of reserved memory regions since these require
>>> system software to create holes in the DMA virtual address range
>>> available to system software and its driver."
>>
>> Nice that you quote it, but did you also read it properly? There's this
>> little "etc" following the explicit naming of USB and UMA...
> 
> Yes. But this already clarify RMRR "used for legacy usage" and "avoid or 
> limit use of reserved memory regions", so RMRR would be gone finally. So 
> I mean it may be acceptable to assume something based our known info.

No. Making assumption on observed broken behavior is okay (to work
around it), but making assumptions for not (yet) observed correct
behavior to be absent should never be done.

>>>> In the tool stack, don't even populate these holes with RAM. This
>>>> will then lead to RAM getting populated further up at the upper end.
>>>
>>> Shouldn't populate RAM still with guest_physmap_add_entry()? If yes, we
>>> already be there to mark them as p2m_access_n.
>>
>> Marking them with p2m_access_n is not the same as not populating
>> the regions in the first place. Again - hiding multiple megabytes of
>> memory (and who knows if it can't grow into the gigabyte range) is
>> just not acceptable. Even for just a few pages I wouldn't be really
> 
> I don't think so. If you're considering a VM, this case should be same 
> under native circumstance. And in native case, all RMRR ranges are 
> marked as reserved in e820 table.

That would only be a valid comparison if all devices associated with
RMRRs would also be passed to that particular VM. But since you're
doing the E820 adjustment to all VMs (no matter whether they would
ever get _any_ device passed through to them) this is not comparing
like entities.

Thinking about this some more, this odd universal hole punching in
the E820 is very likely to end up causing problems. Hence I think
this really should be optional behavior, with pass through of devices
associated with RMRRs failing if not used. (This ought to include
punching holes for _just_ the devices passed through to a guest
upon creation when the option is not enabled.)

Jan


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