Re: [PATCH v2 3/6] x86/mtrr: Fix Xorg crashes in Qemu sessions

[Borislav Petkov]

On Wed, Mar 16, 2016 at 06:46:56PM -0600, Toshi Kani wrote:
> A Xorg failure on qemu32 was reported as a regression caused
> by 'commit 9cd25aac1f44 ("x86/mm/pat: Emulate PAT when it is
> disabled")'. [1]  This patch fixes the regression.

I hope so.

> Negative effects of this regression were two failures in Xorg
> on qemu32 env, which were triggered by the fact that its virtual

"... with QEMU CPU model "qemu32" (-cpu qemu32) ... "

> CPU does not support MTRR. [2]
> 
>  #1. copy_process() failed in the check in reserve_pfn_range()
> 
>     copy_process
>      copy_mm
>       dup_mm
>        dup_mmap
>         copy_page_range
>          track_pfn_copy
>           reserve_pfn_range

Here's where you describe why it failed the check and which check.

> 
>  #2. error path in copy_process() then hit WARN_ON_ONCE in
>      untrack_pfn().
> 
>      x86/PAT: Xorg:509 map pfn expected mapping type uncached-
>      minus for [mem 0xfd000000-0xfdffffff], got write-combining
>       Call Trace:
>      dump_stack+0x58/0x79
>      warn_slowpath_common+0x8b/0xc0
>      ? untrack_pfn+0x9f/0xb0
>      ? untrack_pfn+0x9f/0xb0
>      warn_slowpath_null+0x22/0x30
>      untrack_pfn+0x9f/0xb0
>      ? __kunmap_atomic+0x54/0x110
>      unmap_single_vma+0x56f/0x580
>      ? pagevec_move_tail_fn+0xa0/0xa0
>      unmap_vmas+0x43/0x60
>      exit_mmap+0x5f/0xf0
>      mmput+0x2d/0xa0
>      copy_process.part.47+0x1229/0x1430
>      _do_fork+0xb4/0x3b0
>      SyS_clone+0x2c/0x30
>      do_syscall_32_irqs_on+0x54/0xb0
>      entry_INT80_32+0x2a/0x2a

You can delete the offsets after the "+" - they're useless.

> These negative effects are caused by two separate bugs, but they
> can be dealt in lower priority.

??

> Fixing the pat_init() issue below
> avoids Xorg to hit these cases.
> 
> When the CPU does not support MTRR, MTRR does not call pat_init(),
> which leaves PAT enabled without initializing PAT.  This pat_init()
> issue is a long-standing issue, but manifested as issue #1 (and then
> hit issue #2) with the commit

commit 9cd25aac1f44 ?

> because the memtype now tracks cache
> attribute with 'page_cache_mode'.  A WC map request is tracked as WC
> in memtype, but sets a PTE as UC (pgprot) per __cachemode2pte_tbl[].
> This caused the error in reserve_pfn_range() when it was called from
> track_pfn_copy(), which obtained pgprot from a PTE.  It converts
> pgprot to page_cache_mode, which does not necessarily result in
> the original page_cache_mode since __cachemode2pte_tbl[] redirects
> multiple types to UC.  This is a separate issue in reserve_pfn_range().

Good.

> This pat_init() issue existed before the commit, but we used pgprot
> in memtype.  Hence, we did not have issue #1 before.  But WC request
> resulted in WT in effect because WC pgrot is actually WT when PAT
> is not initialized.  This is not how it was designed to work.  When
> PAT is set to disable properly, WC is converted to UC.  The use of
> WT can result in a system crash if the target range does not support
> WT.  Fortunately, nobody ran into such issue before.
> 
> To fix this pat_init() issue, PAT code has been enhanced to provide
> pat_disable() interface, which disables the OS to initialize PAT MSR,

				... prevents the OS from initializing the PAT MSR.

> and sets PAT table to the BIOS handoff state.

> This patch changes
> MTRR code to call pat_disable() when MTRR is disabled as PAT cannot
> be initialized in this case.  This sets PAT to disable properly, and
> makes PAT code to bypass the memtype check.  This avoids issue #1
> (which can be dealt in lower priority).

You don't need all that text from "This patch ..." on - we can see that
in the diff. The commit message needs to contain "why" not "what".

> 
> [1]: https://lkml.org/lkml/2016/3/3/828
> [2]: https://lkml.org/lkml/2016/3/4/775

I believe I already mentioned that links should be supplied like this:

Link: http://lkml.kernel.org/r/<Message-ID>

> Signed-off-by: Toshi Kani <toshi.kani@xxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxx>
> Cc: Luis R. Rodriguez <mcgrof@xxxxxxxx>
> Cc: Juergen Gross <jgross@xxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/mtrr.h     |    6 +++++-
>  arch/x86/kernel/cpu/mtrr/main.c |   10 +++++++++-
>  2 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/mtrr.h b/arch/x86/include/asm/mtrr.h
> index b94f6f6..634c593 100644
> --- a/arch/x86/include/asm/mtrr.h
> +++ b/arch/x86/include/asm/mtrr.h
> @@ -24,6 +24,7 @@
>  #define _ASM_X86_MTRR_H
>  
>  #include <uapi/asm/mtrr.h>
> +#include <asm/pat.h>
>  
>  
>  /*
> @@ -83,9 +84,12 @@ static inline int mtrr_trim_uncached_memory(unsigned long end_pfn)
>  static inline void mtrr_centaur_report_mcr(int mcr, u32 lo, u32 hi)
>  {
>  }
> +static inline void mtrr_bp_init(void)
> +{
> +	pat_disable("Skip PAT initialization");

Make that more user-friendly:

		   "MTRRs disabled, skipping PAT initialization too."

> +}
>  
>  #define mtrr_ap_init() do {} while (0)
> -#define mtrr_bp_init() do {} while (0)
>  #define set_mtrr_aps_delayed_init() do {} while (0)
>  #define mtrr_aps_init() do {} while (0)
>  #define mtrr_bp_restore() do {} while (0)
> diff --git a/arch/x86/kernel/cpu/mtrr/main.c b/arch/x86/kernel/cpu/mtrr/main.c
> index 10f8d47..2d7d8d7 100644
> --- a/arch/x86/kernel/cpu/mtrr/main.c
> +++ b/arch/x86/kernel/cpu/mtrr/main.c
> @@ -759,8 +759,16 @@ void __init mtrr_bp_init(void)
>  		}
>  	}
>  
> -	if (!mtrr_enabled())
> +	if (!mtrr_enabled()) {
>  		pr_info("MTRR: Disabled\n");
> +
> +		/*
> +		 * PAT initialization relies on MTRR's rendezvous handler.
> +		 * Skip PAT init until the handler can initialize both
> +		 * features independently.
> +		 */
> +		pat_disable("Skip PAT initialization");

Ditto: you can merge the pr_info text with the pat_disable() string.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix ImendÃrffer, Jane Smithard, Graham Norton, HRB 21284 (AG NÃrnberg)
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