Messages in this thread | | | Date | Tue, 6 Jun 2017 19:21:38 -0400 (EDT) | From | Mikulas Patocka <> | Subject | Re: [PATCH v2] X86: don't report PAT on CPUs that don't support it |
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On Tue, 6 Jun 2017, Andy Lutomirski wrote:
> On Tue, Jun 6, 2017 at 3:49 PM, Mikulas Patocka <mpatocka@redhat.com> wrote: > > > > > > On Sun, 28 May 2017, Andy Lutomirski wrote: > > > >> On Sun, May 28, 2017 at 11:18 AM, Bernhard Held <berny156@gmx.de> wrote: > >> > Hi, > >> > > >> > this patch breaks the boot of my kernel. The last message is "Booting > >> > the kernel.". > >> > > >> > My setup might be unusual: I'm running a Xenon E5450 (LGA 771) in a > >> > Gigbayte G33-DS3R board (LGA 775). The BIOS is patched with the > >> > microcode of the E5450 and recognizes the CPU. > >> > > >> > Please find below the dmesg of a the latest kernel w/o the PAT-patch. > >> > I'm happy to provide more information or to test patches. > >> > >> I think this patch is bogus. pat_enabled() sure looks like it's > >> supposed to return true if PAT is *enabled*, and these days PAT is > >> "enabled" even if there's no HW PAT support. Even if the patch were > >> somehow correct, it should have been split up into two patches, one to > >> change pat_enabled() and one to use this_cpu_has(). > >> > >> Ingo, I'd suggest reverting the patch, cc-ing stable on the revert so > >> -stable knows not to backport it, and starting over with the fix. > >> >From very brief inspection, the right fix is to make sure that > >> pat_init(), or at least init_cache_modes(), gets called on the > >> affected CPUs. > >> > >> --Andy > > > > Hi > > > > Here I send the second version of the patch. It drops the change from > > boot_cpu_has(X86_FEATURE_PAT) to this_cpu_has(X86_FEATURE_PAT) (that > > caused kernel to be unbootable for some people). > > > > Another change is that setup_arch() calls init_cache_modes() if PAT is > > disabled, so that init_cache_modes() is always called. > > > > Mikulas > > > > > > > > From: Mikulas Patocka <mpatocka@redhat.com> > > > > In the file arch/x86/mm/pat.c, there's a variable __pat_enabled. The > > variable is set to 1 by default and the function pat_init() sets > > __pat_enabled to 0 if the CPU doesn't support PAT. > > > > However, on AMD K6-3 CPU, the processor initialization code never calls > > pat_init() and so __pat_enabled stays 1 and the function pat_enabled() > > returns true, even though the K6-3 CPU doesn't support PAT. > > > > The result of this bug is that this warning is produced when attemting to > > start the Xserver and the Xserver doesn't start (fork() returns ENOMEM). > > Another symptom of this bug is that the framebuffer driver doesn't set the > > K6-3 MTRR registers. > > > > This patch changes pat_enabled() so that it returns true only if pat > > initialization was actually done. > > Why? Shouldn't calling init_cache_modes() be sufficient? > > --Andy
See the function arch_phys_wc_add():
if (pat_enabled() || !mtrr_enabled()) return 0; /* Success! (We don't need to do anything.) */ ret = mtrr_add(base, size, MTRR_TYPE_WRCOMB, true);
- if pat_enabled() returns true, that function doesn't set MTRRs. pat_enabled() must return false on systems without PAT, so that MTRRs are set.
Mikulas
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