[PATCH v2 04/37] powerpc/64s: remove KVM SKIP test from instruction breakpoint handler

Nicholas Piggin npiggin at gmail.com
Sat Feb 27 10:51:47 AEDT 2021


Excerpts from Daniel Axtens's message of February 26, 2021 3:44 pm:
> Nicholas Piggin <npiggin at gmail.com> writes:
> 
>> The code being executed in KVM_GUEST_MODE_SKIP is hypervisor code with
>> MSR[IR]=0, so the faults of concern are the d-side ones caused by access
>> to guest context by the hypervisor.
>>
>> Instruction breakpoint interrupts are not a concern here. It's unlikely
>> any good would come of causing breaks in this code, but skipping the
>> instruction that caused it won't help matters (e.g., skip the mtmsr that
>> sets MSR[DR]=0 or clears KVM_GUEST_MODE_SKIP).
> 
> I'm not entirely clear on the example here, but the patch makes sense
> and I can follow your logic for removing the IKVM_SKIP handler from the
> instruction breakpoint exception.

The example just means that a breakpoint interrupt on any instruction 
inside the guest mode skip region would be skipped, and skipping one of 
those (mtmsrd or store that gets us out of guest mode skip) would cause 
a crash.

Thanks,
Nick

> 
> On that basis:
> Reviewed-by: Daniel Axtens <dja at axtens.net>
> 
> Kind regards,
> Daniel
> 
>>
>> Reviewed-by: Fabiano Rosas <farosas at linux.ibm.com>
>> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
>> ---
>>  arch/powerpc/kernel/exceptions-64s.S | 1 -
>>  1 file changed, 1 deletion(-)
>>
>> diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
>> index a027600beeb1..0097e0676ed7 100644
>> --- a/arch/powerpc/kernel/exceptions-64s.S
>> +++ b/arch/powerpc/kernel/exceptions-64s.S
>> @@ -2553,7 +2553,6 @@ EXC_VIRT_NONE(0x5200, 0x100)
>>  INT_DEFINE_BEGIN(instruction_breakpoint)
>>  	IVEC=0x1300
>>  #ifdef CONFIG_KVM_BOOK3S_PR_POSSIBLE
>> -	IKVM_SKIP=1
>>  	IKVM_REAL=1
>>  #endif
>>  INT_DEFINE_END(instruction_breakpoint)
>> -- 
>> 2.23.0
> 


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