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Date: Thu, 23 May 2024 19:03:21 +0200
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Cc: alexandre.chartre@oracle.com, linux-kernel@vger.kernel.org,
        daniel.sneddon@linux.intel.com, pawan.kumar.gupta@linux.intel.com,
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Subject: Re: [PATCH] x86/bhi: BHI mitigation can trigger warning in #DB
 handler
Content-Language: en-US
To: Dave Hansen <dave.hansen@intel.com>, x86@kernel.org, kvm@vger.kernel.org
References: <20240523123322.3326690-1-alexandre.chartre@oracle.com>
 <a04d82be-a0d6-4e53-b47c-dba8402199e7@intel.com>
 <1c69f62e-0dee-4caa-9cbe-f43d8efd597b@oracle.com>
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From: Alexandre Chartre <alexandre.chartre@oracle.com>
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On 5/23/24 17:36, Dave Hansen wrote:
> On 5/23/24 07:52, Alexandre Chartre wrote:
>>> Should we wrap up this gem and put it with the other entry selftests?
>>
>> It looks like tools/testing/selftests/x86/single_step_syscall.c tests
>> sysenter with TF set but it doesn't check if the kernel issues any
>> warning.
> 
> Does it actually trip the warning though? I'm a bit surprised that
> nobody reported it if so.

single_step_syscall does trigger the warning:

$ ./single_step_syscall
[RUN]	Set TF and check nop
[OK]	Survived with TF set and 26 traps
[RUN]	Set TF and check syscall-less opportunistic sysret
[OK]	Survived with TF set and 30 traps
[RUN]	Set TF and check a fast syscall
[OK]	Survived with TF set and 40 traps
[RUN]	Fast syscall with TF cleared
[OK]	Nothing unexpected happened
[RUN]	Set TF and check SYSENTER
	Got SIGSEGV with RIP=ed7fe579, TF=256
[RUN]	Fast syscall with TF cleared
[OK]	Nothing unexpected happened

On the console:

[ 1546.656252] WARNING: CPU: 124 PID: 8413 at arch/x86/kernel/traps.c:1009 exc_debug_kernel+0xd2/0x160
..
[ 1546.656352] RIP: 0010:clear_bhb_loop+0x0/0xb0


alex.