[Re: [poky] [PATCH 1/1] poky: update qemu* to prefer 4.4 kernel] On 13/02/2016
(Sat 17:17) Richard Purdie wrote:
> I'm moving the discussion to OE-Core and pulling in some kernel people.
> I think I understand what is wrong and how to fix it but I could use
> someone who actually knows this code.
>
> To summarise the story so far, 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
So Bruce helped me set up a reproducer locally today since he'd already
invested the time on that, and then I boiled that down to divorce it
from the slower steps of build-deploy-boot to make the bisect something
that mortal humans could tolerate.
Amusingly enough that led to:
commit 9cd25aac1f44f269de5ecea11f7d927f37f1d01c
Author: Borislav Petkov <b...@suse.de>
Date: Thu Jun 4 18:55:10 2015 +0200
x86/mm/pat: Emulate PAT when it is disabled
So while some of us were joking on IRC about the validity of forcibly
disabling PAT (via cmdline or Kconfig) as a workaround, the one line
shortlog above tells us that it wasn't so off the mark after all.
Bruce and I will decide what to do with this tomorrow, but since Richard
spent so much time on it, I thought he'd like to know this in the
interim. Good times. :-/
Paul.
--
>
> ------------[ 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.
>
> I can confirm that if I make that change, X boots up just fine.
>
> The problem is I really have no idea what I'm doing :).
>
> Could someone who understands this code have a look and see whether the
> above makes sense and if it does, perhaps open a discussion with
> upstream about how to fix this properly (assuming my change isn't
> actually the correct fix)?
>
> We don't see this on qemux86-64 since that has more PAT bits working
> and hence the values map correctly.
>
> Bruce: Would you accept a patch doing the above for now?
>
> Cheers,
>
> Richard
>
>
--
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