From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:60097)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1anTDF-0003Gk-ON
	for qemu-devel@nongnu.org; Tue, 05 Apr 2016 11:48:18 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1anTDD-0003Ti-0o
	for qemu-devel@nongnu.org; Tue, 05 Apr 2016 11:48:17 -0400
Received: from mx1.redhat.com ([209.132.183.28]:51000)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1anTDC-0003Te-RG
	for qemu-devel@nongnu.org; Tue, 05 Apr 2016 11:48:14 -0400
References: <1459834253-8291-1-git-send-email-cota@braap.org>
	<1459834253-8291-8-git-send-email-cota@braap.org>
	<5703DCB7.50302@twiddle.net>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <5703DE37.3080306@redhat.com>
Date: Tue, 5 Apr 2016 17:48:07 +0200
MIME-Version: 1.0
In-Reply-To: <5703DCB7.50302@twiddle.net>
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH 07/10] tb hash: hash phys_pc, pc,
	and flags with xxhash
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Richard Henderson <rth@twiddle.net>, "Emilio G. Cota" <cota@braap.org>, QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@greensocs.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Sergey Fedorov <serge.fdrv@gmail.com>, =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, Peter Crosthwaite <crosthwaite.peter@gmail.com>



On 05/04/2016 17:41, Richard Henderson wrote:
> 
>> +static inline
>> +uint32_t tb_hash_func(tb_page_addr_t phys_pc, target_ulong pc,
>> uint64_t flags)
>>   {
>> -    return (pc >> 2) & (CODE_GEN_PHYS_HASH_SIZE - 1);
>> +    struct key {
>> +        tb_page_addr_t phys_pc;
>> +        target_ulong pc;
>> +        uint64_t flags;
> 
> The flags field should be uint32_t, not uint64_t.
> See its definition in TranslationBlock.
> 
>> +    } QEMU_PACKED k;
>> +    unsigned int hash;
>> +
>> +    k.phys_pc = phys_pc;
>> +    k.pc = pc;
>> +    k.flags = flags;
>> +    hash = qemu_xxh32((uint32_t *)&k, sizeof(k) / sizeof(uint32_t), 1);
> 
> I'm less than happy about putting data into a struct and hashing that
> memory. Why don't you just take the ideas from xxh32/xxh64 and perform
> the hash directly right here?
> 
> In particular, you've 3 data elements here, a maximum of 5 32-bit words,
> thus the loop within xxh32 will never roll, so it's mostly dead code.

I think it's fine to use the struct.  The exact size of the struct
varies from 3 to 5 32-bit words, so it's hard to write nice
size-dependent code for the hash.

Paolo