3 files changed, 103 insertions, 1 deletions

diff --git a/meta/recipes-kernel/linux/linux-yocto/0001-Fix-qemux86-pat-issue.patch b/meta/recipes-kernel/linux/linux-yocto/0001-Fix-qemux86-pat-issue.patch
new file mode 100644
index 0000000000..367d9b4ace
--- /dev/null
+++ b/meta/recipes-kernel/linux/linux-yocto/0001-Fix-qemux86-pat-issue.patch
@@ -0,0 +1,100 @@
+From 9bfb2c711f66fc8eb6482f5ca6ea33c2ec1d792f Mon Sep 17 00:00:00 2001
+From: Richard Purdie <richard.purdie@linuxfoundation.org>
+Date: Sat, 13 Feb 2016 17:36:23 +0000
+Subject: [PATCH] Fix qemux86 pat issue
+
+on qemux86, X doesn't start and there is
+a backtrace in the logs:
+
+x86/PAT: Xorg:705 map pfn expected mapping type uncached-minus for [mem 0xfd000000-0xfdffffff], got write-combining
+------------[ cut here ]------------
+WARNING: CPU: 0 PID: 705 at /media/build1/poky/build/tmp/work-shared/qemux86/kernel-source/arch/x86/mm/pat.c:985 untrack_pfn+0xaf/0xc0()
+Modules linked in: uvesafb
+CPU: 0 PID: 705 Comm: Xorg Not tainted 4.4.1-yocto-standard #1
+Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.8.2-0-g33fbe13 by qemu-project.org 04/01/2014
+ 00000000 00000000 cf14dd78 c1397ab2 00000000 cf14dda8 c1051477 c1aa4f6c
+ 00000000 000002c1 c1a9fa4c 000003d9 c104b98f c104b98f cf244000 b6355000
+ 00000000 cf14ddb8 c1051552 00000009 00000000 cf14dde0 c104b98f cf14ddd0
+Call Trace:
+ [<c1397ab2>] dump_stack+0x4b/0x79
+ [<c1051477>] warn_slowpath_common+0x87/0xc0
+ [<c104b98f>] ? untrack_pfn+0xaf/0xc0
+ [<c104b98f>] ? untrack_pfn+0xaf/0xc0
+ [<c1051552>] warn_slowpath_null+0x22/0x30
+ [<c104b98f>] untrack_pfn+0xaf/0xc0
+ [<c104d54c>] ? kmap_atomic_prot+0x3c/0xf0
+ [<c114e17f>] unmap_single_vma+0x4ef/0x500
+ [<c114f007>] unmap_vmas+0x37/0x50
+ [<c1154f8f>] exit_mmap+0x5f/0xf0
+ [<c104eedd>] mmput+0x2d/0xb0
+ [<c105009c>] copy_process+0xd2c/0x13c0
+ [<c1050892>] _do_fork+0x82/0x340
+ [<c105f2d1>] ? SyS_rt_sigaction+0x51/0xa0
+ [<c1050c3c>] SyS_clone+0x2c/0x30
+ [<c1001a03>] do_syscall_32_irqs_on+0x53/0xb0
+ [<c189a94a>] entry_INT80_32+0x2a/0x2a
+---[ end trace be3e0a61097feddc ]---
+x86/PAT: Xorg:705 map pfn expected mapping type uncached-minus for [mem 0xfd000000-0xfdffffff], got write-combining
+
+The entry in question is setup by uvesafb which in its
+uvesafb_ioremap() function calls ioremap_wc().
+
+It appears that Xorg mmaps this from userspace, then later does a
+fork() to execute a utility. At this point, when creating the vmas for
+the new process, the pat code says "eeek!" as the protection mode for
+the new vmas don't match the old one, returns -EINVAL, the process dies
+and X goes with it.
+
+There are a few hammers we can hit this with, we can boot with "nopat"
+option which makes the problem go away, or turn off CONFIG_X86_PAT. No
+surprises there. Changing uvesafb to use mtrr=0 doesn't help since the
+ioremap_wc call still happens.
+
+The real issue is the "expected mapping type uncached-minus for got
+write-combining" message, it all goes wrong from there.
+
+Upon looking at the code and scratching my head for a long while, I
+notice that there are two ways of representing the protection mode
+data, "enum page_cache_mode" and "pgprot_t & _PAGE_CACHE_MASK".
+
+The exact meaning of pgprot_t depends on which CPU you're running,
+older CPUs have errata meaning only a small number of bits can be used.
+The exact mapping table is determined by __cachemode2pte_tbl and is
+updated at boot by calls from update_cache_mode_entry().
+
+The result of this if you map enum -> pgprot_t, then try to do pgprot_t
+-> enum, you can get different values since its not a 1:1 mapping.
+
+This means the comparison in reserve_pfn_range() where it does "pcm !=
+want_pcm" isn't correct and can trigger even in cases where there isn't
+a problem.
+
+This can be "fixed" by doing cachemode2protval(pcm) !=
+cachemode2protval(want_pcm) and checking whether the protection bits
+match, rather than the enum values, since in reality this is what we
+really care about. With that change, X boots up just fine.
+
+We don't see this on qemux86-64 since that has more PAT bits working
+and hence the values map correctly.
+
+Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
+---
+ arch/x86/mm/pat.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
+index 031782e..11064fb 100644
+--- a/arch/x86/mm/pat.c
++++ b/arch/x86/mm/pat.c
+@@ -833,7 +833,7 @@ static int reserve_pfn_range(u64 paddr, unsigned long size, pgprot_t *vma_prot,
+        if (ret)
+                return ret;
+ 
+-       if (pcm != want_pcm) {
++       if (cachemode2protval(pcm) != cachemode2protval(want_pcm)) {
+                if (strict_prot ||
+                    !is_new_memtype_allowed(paddr, size, want_pcm, pcm)) {
+                        free_memtype(paddr, paddr + size);
+-- 
+2.5.0
+
diff --git a/meta/recipes-kernel/linux/linux-yocto_4.4.bb b/meta/recipes-kernel/linux/linux-yocto_4.4.bb
index 1beeea6698..c0edcf13c3 100644
--- a/meta/recipes-kernel/linux/linux-yocto_4.4.bb
+++ b/meta/recipes-kernel/linux/linux-yocto_4.4.bb
@@ -40,3 +40,5 @@ KERNEL_FEATURES_append_qemuall=" cfg/virtio.scc"
 KERNEL_FEATURES_append_qemux86=" cfg/sound.scc cfg/paravirt_kvm.scc"
 KERNEL_FEATURES_append_qemux86-64=" cfg/sound.scc cfg/paravirt_kvm.scc"
 KERNEL_FEATURES_append = " ${@bb.utils.contains("TUNE_FEATURES", "mx32", " cfg/x32.scc", "" ,d)}"
+
+SRC_URI_append = " file://0001-Fix-qemux86-pat-issue.patch"
diff --git a/scripts/runqemu-internal b/scripts/runqemu-internal
index ad854d108d..ebed2bdd01 100755
--- a/scripts/runqemu-internal
+++ b/scripts/runqemu-internal
@@ -434,7 +434,7 @@ if [ "$MACHINE" = "qemux86" ]; then
     fi
     # Currently oprofile's event based interrupt mode doesn't work(Bug #828) in
     # qemux86 and qemux86-64. We can use timer interrupt mode for now.
-    KERNCMDLINE="$KERNCMDLINE oprofile.timer=1 nopat"
+    KERNCMDLINE="$KERNCMDLINE oprofile.timer=1"
 fi
 
 if [ "$MACHINE" = "qemux86-64" ]; then