Re: [PATCH 1/2] x86: ioremap: fix wrong physical address handling

[Kenji Kaneshige]

(2010/06/17 18:35), Jeremy Fitzhardinge wrote:
> On 06/17/2010 07:03 AM, H. Peter Anvin wrote:
> > On 06/16/2010 09:55 PM, Kenji Kaneshige wrote:
> >
> > > > I think they might be. Kenji?
> > > No. My addresses are in the 44-bits range (around fc000000000). So it is
> > > not required for my problem. This change assumes that phys_addr can be
> > > above 44-bits (up to 52-bits (and higher in the future?)).
> > >
> > > By the way, is there linux kernel limit regarding above 44-bits physical
> > > address in x86_32 PAE? For example, pfn above 32-bits is not supported?
>
> That's an awkward situation.  I would tend to suggest that you not
> support this type of machine with a 32-bit kernel.  Is it a sparse
> memory system, or is there a device mapped in that range?

Device mapped range in my case.
Fortunately, the address is in 44-bits range. I'd like to focus on
making 2^44 work correctly this time.

Thanks,
Kenji Kaneshige




> I guess it would be possible to special-case ioremap to allow the
> creation of such mappings, but I don't know what kind of system-wide
> fallout would happen as a result.  The consequences of something trying
> to extract a pfn from one of those ptes would be
>
> > There are probably places at which PFNs are held in 32-bit numbers,
> > although it would be good to track them down if it isn't too expensive
> > to fix them (i.e. doesn't affect generic code.)
>
> There are many places which hold pfns in 32 bit variables on 32 bit
> systems; the standard type for pfns is "unsigned long", pretty much
> everywhere in the kernel.  It might be worth defining a pfn_t and
> converting usage over to that, but it would be a pervasive change.
>
> > This also affects paravirt systems, i.e. right now Xen has to locate all
> > 32-bit guests below 64 GB, which limits its usefulness.
>
> I don't think the limit is 64GB.  A 32-bit PFN limits us to 2^44, which
> is 16TB.  (32-bit PV Xen guests have another unrelated limit of around
> 160GB physical memory because that as much m2p table will fit into the
> Xen hole in the kernel mapping.)
>
> > > #ifdef CONFIG_X86_PAE
> > > /* 44=32+12, the limit we can fit into an unsigned long pfn */
> > > #define __PHYSICAL_MASK_SHIFT   44
> > > #define __VIRTUAL_MASK_SHIFT    32
> > >
> > > If there is 44-bits physical address limit, I think it's better to use
> > > PHYSICAL_PAGE_MASK for masking physical address, instead of "(phys_addr
> > >
> > > > > PAGE_SHIFT)<<  PAGE_SHIFT)". The PHYSICAL_PAGE_MASK would become
> > > greater value when 44-bits physical address limit is eliminated. And
> > > maybe we need to change phys_addr_valid() returns error if physical
> > > address is above (1<<  __PHYSICAL_MASK_SHIFT)?
> > The real question is how much we can fix without an unreasonable cost.
>
> I think it would be a pretty large change.  From the Xen's perspective,
> any machine even approximately approaching the 2^44 limit will be
> capable of running Xen guests in hvm mode, so PV isn't really a concern.
>
>      J


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