Re: [PATCH] x86: create array based interface to change page attribute

[Arjan van de Ven]

Thomas Hellström wrote:
> Arjan van de Ven wrote:
> > Thomas Hellström wrote:
> >
> > > But what mappings are there,  immediately after alloc_page(), that 
> > > set_memory_np won't catch? 
> >
> > For example on x86 64bit, the kernel text is mapped (to allow 
> > relocatable) in another
> > space as well.. and to allow 2Mb tlbs for this, that second mapping is 
> > bigger than it strictly needs to be.
> > So your alloc_page() could get a page from the free pool that comes 
> > from the pages that have a second mapping
> > due to this rounding.
> > (and more fun, since it's close to frequently accessed memory, the hw 
> > prefetchers may actually just decide to pull
> > such pages into the cache preemptively)
> >
> >
> > > Drivers relying on set_memory_uc touching all mappings the driver 
> > > hasn't set up itself must then have the same problem and needs to be 
> > > fixed; referring in particular to agpgart for which driver the old 
> > > CPA functionality was once created, IIRC.
> >
> > "uc" is different than "np"; for "uc" the implementation, if your cpu 
> > needs this, will fix
> > up the shadow text mappings already today.
> >
> > for "present" this doesn't make sense, since no cpu needs this.
> If this is the checkalias() thingy in x86/pageattr.c, it looks like it 
> doesn't handle np different from the uc case. It does obviously skip NX 
> bit manipulations, though.
>
> Anyway, a more direct question: If we were to fix whatever's missing for 
> the code to fixup the x86-64 shadow text mapping, would you be opposing 
> this way 

I do really oppose set_memory_np/p to work on anything but the mapping it is handled.
that _uc and co touch other mappings is a cpu specific property and an implementation
detail to the API. If/When CPUs exist that don't have issues with aliases, those CPUs
will not change the other mappings.

> to fix the long standing uc/wc aliasing issue, provided we

I'm not opposed to a real fix. I am opposed to a bad hack.

> don't hit any other problems, like clflush() refusing to run on an NP page?

yes clflush doesn't work on not present pages ;)
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