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

[Elena Ufimtseva <elena.ufimtseva@xxxxxxxxxx>]

On Tue, Jan 27, 2015 at 08:24:36AM +0000, Jan Beulich wrote:
> >>> 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,

Yes, that is "can't".

> 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.

I see, thanks for explanation.
> 
> Jan
> 

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