linux-kernel - Re: [PATCH v3 1/9] powerpc/watchpoint: Fix 512 byte boundary limit

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Date:   Wed, 8 Jul 2020 09:52:45 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Jordan Niethe <jniethe5@...il.com>,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>, mikey@...ling.org,
        apopple@...ux.ibm.com, Paul Mackerras <paulus@...ba.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@....fr>,
        naveen.n.rao@...ux.vnet.ibm.com, peterz@...radead.org,
        jolsa@...nel.org, oleg@...hat.com, fweisbec@...il.com,
        mingo@...nel.org, pedromfc@...ibm.com, miltonm@...ibm.com,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/9] powerpc/watchpoint: Fix 512 byte boundary limit



Le 08/07/2020 à 09:44, Jordan Niethe a écrit :
> On Wed, Jul 8, 2020 at 2:53 PM Ravi Bangoria
> <ravi.bangoria@...ux.ibm.com> wrote:
>>
>> Milton Miller reported that we are aligning start and end address to
>> wrong size SZ_512M. It should be SZ_512. Fix that.
>>
>> While doing this change I also found a case where ALIGN() comparison
>> fails. Within a given aligned range, ALIGN() of two addresses does not
>> match when start address is pointing to the first byte and end address
>> is pointing to any other byte except the first one. But that's not true
>> for ALIGN_DOWN(). ALIGN_DOWN() of any two addresses within that range
>> will always point to the first byte. So use ALIGN_DOWN() instead of
>> ALIGN().
>>
>> Fixes: e68ef121c1f4 ("powerpc/watchpoint: Use builtin ALIGN*() macros")
>> Reported-by: Milton Miller <miltonm@...ibm.com>
>> Signed-off-by: Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
>> ---
>>   arch/powerpc/kernel/hw_breakpoint.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/powerpc/kernel/hw_breakpoint.c b/arch/powerpc/kernel/hw_breakpoint.c
>> index 0000daf0e1da..031e6defc08e 100644
>> --- a/arch/powerpc/kernel/hw_breakpoint.c
>> +++ b/arch/powerpc/kernel/hw_breakpoint.c
>> @@ -419,7 +419,7 @@ static int hw_breakpoint_validate_len(struct arch_hw_breakpoint *hw)
>>          if (dawr_enabled()) {
>>                  max_len = DAWR_MAX_LEN;
>>                  /* DAWR region can't cross 512 bytes boundary */
>> -               if (ALIGN(start_addr, SZ_512M) != ALIGN(end_addr - 1, SZ_512M))
>> +               if (ALIGN_DOWN(start_addr, SZ_512) != ALIGN_DOWN(end_addr - 1, SZ_512))
> I wonder if you should use end_addr - 1, but rather end_addr. For example:
> 512 -> 1023, because of the -1, 1024 will now be included in this
> range meaning 513 bytes?

end_addr is not part of the range.

If you want the range [512;1023], it means addr 512 len 512, that is 
end_addr = addr + len = 1024

Christophe