linux-kernel - Re: [PATCH v2 13/16] powerpc/watchpoint: Prepare handler to handle more than one watcnhpoint

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Message-ID: <78559ff4-c2c3-e652-a906-8f40673b53d6@c-s.fr>
Date:   Wed, 1 Apr 2020 11:20:10 +0200
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
Cc:     mpe@...erman.id.au, mikey@...ling.org, apopple@...ux.ibm.com,
        paulus@...ba.org, npiggin@...il.com,
        naveen.n.rao@...ux.vnet.ibm.com, peterz@...radead.org,
        jolsa@...nel.org, oleg@...hat.com, fweisbec@...il.com,
        mingo@...nel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 13/16] powerpc/watchpoint: Prepare handler to handle
 more than one watcnhpoint



Le 01/04/2020 à 11:13, Ravi Bangoria a écrit :
> 
> 
> On 4/1/20 12:20 PM, Christophe Leroy wrote:
>>
>>
>> Le 01/04/2020 à 08:13, Ravi Bangoria a écrit :
>>> Currently we assume that we have only one watchpoint supported by hw.
>>> Get rid of that assumption and use dynamic loop instead. This should
>>> make supporting more watchpoints very easy.
>>>
>>> With more than one watchpoint, exception handler need to know which
>>> DAWR caused the exception, and hw currently does not provide it. So
>>> we need sw logic for the same. To figure out which DAWR caused the
>>> exception, check all different combinations of user specified range,
>>> dawr address range, actual access range and dawrx constrains. For ex,
>>> if user specified range and actual access range overlaps but dawrx is
>>> configured for readonly watchpoint and the instruction is store, this
>>> DAWR must not have caused exception.
>>>
>>> Signed-off-by: Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
>>> ---
>>>   arch/powerpc/include/asm/processor.h |   2 +-
>>>   arch/powerpc/include/asm/sstep.h     |   2 +
>>>   arch/powerpc/kernel/hw_breakpoint.c  | 396 +++++++++++++++++++++------
>>>   arch/powerpc/kernel/process.c        |   3 -
>>>   4 files changed, 313 insertions(+), 90 deletions(-)
>>>
>>
>> [...]
>>
>>> -static bool
>>> -dar_range_overlaps(unsigned long dar, int size, struct 
>>> arch_hw_breakpoint *info)
>>> +static bool dar_user_range_overlaps(unsigned long dar, int size,
>>> +                    struct arch_hw_breakpoint *info)
>>>   {
>>>       return ((dar <= info->address + info->len - 1) &&
>>>           (dar + size - 1 >= info->address));
>>>   }
>>
>> Here and several other places, I think it would be more clear if you 
>> could avoid the - 1 :
>>
>>      return ((dar < info->address + info->len) &&
>>          (dar + size > info->address));
> 
> Ok. see below...
> 
>>
>>
>>> +static bool dar_in_hw_range(unsigned long dar, struct 
>>> arch_hw_breakpoint *info)
>>> +{
>>> +    unsigned long hw_start_addr, hw_end_addr;
>>> +
>>> +    hw_start_addr = ALIGN_DOWN(info->address, HW_BREAKPOINT_SIZE);
>>> +    hw_end_addr = ALIGN(info->address + info->len, 
>>> HW_BREAKPOINT_SIZE) - 1;
>>> +
>>> +    return ((hw_start_addr <= dar) && (hw_end_addr >= dar));
>>> +}
>>
>>      hw_end_addr = ALIGN(info->address + info->len, HW_BREAKPOINT_SIZE);
>>
>>      return ((hw_start_addr <= dar) && (hw_end_addr > dar));
> 
> I'm using -1 while calculating end address is to make it
> inclusive. If I don't use -1, the end address points to a
> location outside of actual range, i.e. it's not really an
> end address.

But that's what is done is several places, for instance:

https://elixir.bootlin.com/linux/v5.6/source/arch/powerpc/mm/dma-noncoherent.c#L22

https://elixir.bootlin.com/linux/v5.6/source/arch/powerpc/include/asm/book3s/32/kup.h#L92

In several places like this, end is outside of the range. My feeling is 
that is helps with readability.

Christophe