Messages in this thread | | | From | Jesse Barnes <> | Subject | Re: [git head] Should X86_PAT really default to yes? | Date | Fri, 2 May 2008 15:07:28 -0700 |
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On Friday, May 02, 2008 2:55 pm Pallipadi, Venkatesh wrote: > >-----Original Message----- > >From: Barnes, Jesse > >Sent: Friday, May 02, 2008 1:40 PM > >To: Pallipadi, Venkatesh > >Cc: Frans Pop; linux-kernel@vger.kernel.org; Ingo Molnar; > >Packard, Keith > >Subject: Re: [git head] Should X86_PAT really default to yes? > > > >On Friday, May 02, 2008 12:37 pm Pallipadi, Venkatesh wrote: > >> >-----Original Message----- > >> >From: Frans Pop [mailto:elendil@planet.nl] > >> >Sent: Friday, May 02, 2008 12:22 PM > >> >To: linux-kernel@vger.kernel.org > >> >Cc: Pallipadi, Venkatesh; Ingo Molnar > >> >Subject: [git head] Should X86_PAT really default to yes? > >> > > >> >With X86_PAT enabled, when X is started I get about 40 lines > >> >(with varying > >> >addresses) like: > >> >kernel: Xorg:3358 /dev/mem expected mapping type write-back for > >> >807bf000-81000000, got uncached-minus > > > >These messages? They're coming from the kernel it looks like, > >from the > >map_devmem routine in pat.c. I'm not sure they're accurate > >though; for PCI > >regions /dev/mem is *supposed* to map with UC- and not WB, so > >maybe this > >function needs to be updated? > > Indeed. > I think these messages are due to X using the mprotect workaround to > change UC_MINUS to WB. > I don't see these error messages on my 965 here. May be I have different > x version.
Hm, yeah that could be. strace would tell us.
> > What may be happening: > 1) process A mmaps /dev/mem and gets UC_MINUS > 2) Changes the page table to make pg_prot WB > 3) Does a fork to create process B > 4) While copying the vma, we go through map_devmem request WB, but get > UC_MINUS back > 5) We are not changing vma pg_prot to new value at this point (we should > change this), so one more round of errors will be there when forked > process exits. > > Again, this should not have any side-effect like the band etc. It just a > "friendly warning". It should go away when X moves to using WC or does > not use the mprotect workaround to make pg_prot WB.
More recent versions of X will use sysfs rather than /dev/mem (though we're still using the mprotect hack there to give us UC-), so yeah this warning should already be gone in more recent builds.
Jesse
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