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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
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