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Date:   Mon, 6 Jul 2020 10:57:07 +0200
From:   Jürgen Groß <jgross@...e.com>
To:     Michal Kubecek <mkubecek@...e.cz>,
        Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.org, Andrew Cooper <andrew.cooper3@...rix.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH entry v2 3/6] x86/entry/xen: Route #DB correctly on Xen PV

On 06.07.20 10:41, Michal Kubecek wrote:
> On Fri, Jul 03, 2020 at 10:02:55AM -0700, Andy Lutomirski wrote:
>> On Xen PV, #DB doesn't use IST.  We still need to correctly route it
>> depending on whether it came from user or kernel mode.
>>
>> This patch gets rid of DECLARE/DEFINE_IDTENTRY_XEN -- it was too
>> hard to follow the logic.  Instead, route #DB and NMI through
>> DECLARE/DEFINE_IDTENTRY_RAW on Xen, and do the right thing for #DB.
>> Also add more warnings to the exc_debug* handlers to make this type
>> of failure more obvious.
>>
>> This fixes various forms of corruption that happen when usermode
>> triggers #DB on Xen PV.
>>
>> Fixes: 4c0dcd8350a0 ("x86/entry: Implement user mode C entry points for #DB and #MCE")
>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> ---
>>   arch/x86/include/asm/idtentry.h | 24 ++++++------------------
>>   arch/x86/kernel/traps.c         | 12 ++++++++++++
>>   arch/x86/xen/enlighten_pv.c     | 28 ++++++++++++++++++++++++----
>>   arch/x86/xen/xen-asm_64.S       |  5 ++---
>>   4 files changed, 44 insertions(+), 25 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
>> index cf51c50eb356..94333ac3092b 100644
>> --- a/arch/x86/include/asm/idtentry.h
>> +++ b/arch/x86/include/asm/idtentry.h
> [...]
>> @@ -570,11 +554,15 @@ DECLARE_IDTENTRY_MCE(X86_TRAP_MC,	exc_machine_check);
>>   
>>   /* NMI */
>>   DECLARE_IDTENTRY_NMI(X86_TRAP_NMI,	exc_nmi);
>> -DECLARE_IDTENTRY_XEN(X86_TRAP_NMI,	nmi);
>> +#ifdef CONFIG_XEN_PV
>> +DECLARE_IDTENTRY_RAW(X86_TRAP_NMI,	xenpv_exc_nmi);
>> +#endif
>>   
>>   /* #DB */
>>   DECLARE_IDTENTRY_DEBUG(X86_TRAP_DB,	exc_debug);
>> -DECLARE_IDTENTRY_XEN(X86_TRAP_DB,	debug);
>> +#ifdef CONFIG_XEN_PV
>> +DECLARE_IDTENTRY_RAW(X86_TRAP_DB,	xenpv_exc_debug);
>> +#endif
>>   
>>   /* #DF */
>>   DECLARE_IDTENTRY_DF(X86_TRAP_DF,	exc_double_fault);
> 
> Hello,
> 
> this patch - now in mainline as commit 13cbc0cd4a30 ("x86/entry/32: Fix
> #MC and #DB wiring on x86_32") - seems to break i586 builds with
> CONFIG_XEN_PV=y as xenpv_exc_nmi and xenpv_exc_debug are only defined
> with CONFIG_X86_64:
> 
> [ 1279s] ld: arch/x86/entry/entry_32.o: in function `asm_xenpv_exc_nmi':
> [ 1279s] /home/abuild/rpmbuild/BUILD/kernel-pae-5.8.rc4/linux-5.8-rc4/linux-obj/../arch/x86/include/asm/idtentry.h:557: undefined reference to `xenpv_exc_nmi'
> [ 1279s] ld: arch/x86/entry/entry_32.o: in function `asm_xenpv_exc_debug':
> [ 1279s] /home/abuild/rpmbuild/BUILD/kernel-pae-5.8.rc4/linux-5.8-rc4/linux-obj/../arch/x86/include/asm/idtentry.h:567: undefined reference to `xenpv_exc_debug'

Fix is already queued in tip/tip.git x86/urgent


Juergen

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