From: Julien Grall <julien.grall@arm.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>, xen-devel@lists.xen.org
Cc: sstabellini@kernel.org, Keir Fraser <keir@xen.org>,
andre.przywara@arm.com, Ian Jackson <ian.jackson@eu.citrix.com>,
Tim Deegan <tim@xen.org>, Jan Beulich <jbeulich@suse.com>
Subject: Re: [for-4.7 1/2] xen/bitops: Introduce macros to generate mask
Date: Thu, 14 Apr 2016 09:47:34 +0100 [thread overview]
Message-ID: <570F5926.2090306@arm.com> (raw)
In-Reply-To: <570E8C89.5050101@citrix.com>
Hi Andrew,
On 13/04/2016 19:14, Andrew Cooper wrote:
> On 13/04/16 16:55, Julien Grall wrote:
>> The code has been imported from the header include/linux/bitops.h in
>> Linux v4.6-rc3.
>>
>> Signed-off-by: Julien Grall <julien.grall@arm.com>
>>
>> ---
>> Cc: Ian Jackson <ian.jackson@eu.citrix.com>
>> Cc: Jan Beulich <jbeulich@suse.com>
>> Cc: Keir Fraser <keir@xen.org>
>> Cc: Tim Deegan <tim@xen.org>
>> ---
>> xen/include/xen/bitops.h | 11 +++++++++++
>> 1 file changed, 11 insertions(+)
>>
>> diff --git a/xen/include/xen/bitops.h b/xen/include/xen/bitops.h
>> index cb56f24..e1a4d93 100644
>> --- a/xen/include/xen/bitops.h
>> +++ b/xen/include/xen/bitops.h
>> @@ -3,6 +3,17 @@
>> #include <asm/types.h>
>>
>> /*
>> + * Create a contiguous bitmask starting at bit position @l and ending at
>> + * position @h. For example
>> + * GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000.
>> + */
>> +#define GENMASK(h, l) \
>> + (((~0UL) << (l)) & (~0UL >> (BITS_PER_LONG - 1 - (h))))
>> +
>> +#define GENMASK_ULL(h, l) \
>> + (((~0ULL) << (l)) & (~0ULL >> (BITS_PER_LONG_LONG - 1 - (h))))
>
> You should have just a single GENMASK() which works in terms of LL.
>
> Masks must be signed to work correctly when implicitly extended.
May I ask you why? AFAICT, if the mask is signed, it will result to
duplicate the sign bit to the new bits. This is not the expected
behavior of this macro.
For instance, the following patch is using this macro to mask RES0 bits
in the HPFAR_EL2 register. For ARM, RES0 means the bit is currently read
as zero but the software should not rely on it to preserve forward
compatibility.
Regards,
--
Julien Grall
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next prev parent reply other threads:[~2016-04-14 8:47 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-13 15:55 [for-4.7 0/2] xen/arm: traps: Correctly interpret the content of the register HPFAR_EL2 Julien Grall
2016-04-13 15:55 ` [for-4.7 1/2] xen/bitops: Introduce macros to generate mask Julien Grall
2016-04-13 18:07 ` Stefano Stabellini
2016-04-13 18:14 ` Andrew Cooper
2016-04-14 8:47 ` Julien Grall [this message]
2016-04-14 4:01 ` Jan Beulich
2016-04-14 8:55 ` Julien Grall
2016-04-14 14:56 ` Jan Beulich
2016-04-14 15:08 ` Julien Grall
2016-04-14 15:23 ` Jan Beulich
2016-04-20 12:35 ` Julien Grall
2016-04-20 16:43 ` Jan Beulich
2016-04-22 11:33 ` Julien Grall
2016-04-22 11:49 ` Stefano Stabellini
2016-04-22 15:33 ` Julien Grall
2016-04-22 15:42 ` Stefano Stabellini
2016-04-13 15:55 ` [for-4.7 2/2] xen/arm: traps: Correctly interpret the content of the register HPFAR_EL2 Julien Grall
2016-04-13 18:17 ` Stefano Stabellini
2016-04-13 19:11 ` Tamas K Lengyel
2016-04-13 20:33 ` André Przywara
2016-04-14 17:47 ` [for-4.7 0/2] " Wei Liu
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