Re: [Xen-devel] [PATCH RFC] p2m: p2m_mmio_direct set RW permissions

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.01.15 at 18:30, <elena.ufimtseva@xxxxxxxxxx> wrote:
> On Mon, Jan 26, 2015 at 05:06:12PM +0000, Jan Beulich wrote:
>> >>> On 26.01.15 at 17:57, <elena.ufimtseva@xxxxxxxxxx> wrote:
>> > On Fri, Jan 23, 2015 at 10:50:23AM +0000, Jan Beulich wrote:
>> >> >>> On 22.01.15 at 18:34, <elena.ufimtseva@xxxxxxxxxx> wrote:
>> >> > (XEN)  00000000d56f0000 - 00000000d5fff000 (reserved)
>> >> 
>> >> So this is where one of the RMRRs sits in (and also where
>> >> the faults occur according to the two numbers you sent
>> >> earlier, which - as others have already said - is an indication
>> >> of the reported RMRRs being incomplete), ...
>> >> 
>> >> > (XEN)  00000000d5fff000 - 00000000d6000000 (usable)
>> >> > (XEN)  00000000d7000000 - 00000000df200000 (reserved)
>> >> 
>> >> ... and this is the exact range of the other one. But the usable
>> >> entry between them is a sign of the firmware not doing the
>> >> best job in assigning resources.
>> >> 
>> >> I don't, btw, think that blindly mapping all the reserved regions
>> >> into PVH Dom0's P2M would be (or is, if that's what's happening
>> >> today) correct - these regions are named reserved for a
>> >> reason. In the case here it's actually RAM, not MMIO, and
>> >> Dom0 (as well as Xen) has no business accessing these (for others
>> >> this may be different, e.g. the LAPIC and IO-APIC ones below,
>> >> but Xen learns/knows of them by means different from looking
>> >> at the memory map).
>> > 
>> > I understand this this. At the same time I think pv dom0 does exactly
>> > this blind mapping. I also tried to map these regions as read-only and
>> > that worked. Can be this an option for these ram regions?
>> 
>> No - they're reserved, so there shouldn't be _any_ access to them.
>> The only possible workaround I see as acceptable would be the
>> already proposed addition of a command line option allowing to
>> specify additional RMRR-like regions.
>> 
> 
> Understood. And I am guessing the permissions overloading option should
> be available as well? For example, RW or R only. RMRRs are always mapped 
> with RW.

That's an option, but not a requirement imo.

> Why can be this a platform quirk?

Did you mean "can't"? If not, I don't understand the question. If so,
remember that we're talking about RAM allocated by the firmware
for special purposes. Hence such a quirk would need tailoring to any
particular published firmware version, and would need to take into
account any differences in the memory use that may result from
setting firmware options differently (and assuming that the memory
use is deterministic across boots when the options don't change -
I've seen systems where memory use differed between warm and
cold boots). IOW, not something that's likely to be practical.

Jan


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