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Re: [Qemu-devel] [PATCH 43/47] mirror: allow customizing the granularity

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH 43/47] mirror: allow customizing the granularity
Date: Mon, 30 Jul 2012 16:03:58 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:13.0) Gecko/20120615 Thunderbird/13.0.1

Il 30/07/2012 15:53, Eric Blake ha scritto:
> On 07/30/2012 07:40 AM, Paolo Bonzini wrote:
>> Il 28/07/2012 15:43, Eric Blake ha scritto:
>>>>> +    if (granularity < 512 || granularity > 1048576 * 64) {
>>>>> +        error_set(errp, QERR_INVALID_PARAMETER, device);
>>>>> +        return;
>>>>> +    }
>>>>> +    if (granularity & (granularity - 1)) {
>>>>> +        error_set(errp, QERR_INVALID_PARAMETER, device);
>>>>> +        return;
>>> In the XBZLRE migration series, we decided to round the users input down
>>> to a power of two instead of reject it.  Should we do that here?
>> I can certainly do that, but do you have a pointer to the discussion so
>> that I can understand the rationale?
> https://lists.gnu.org/archive/html/qemu-devel/2012-05/msg02421.html

Hmm, a buffer size (the cache size in the case of XBZRLE) is different
however.  There is no reason in principle why it needs to be a power of
two (except if you know how the hash table is implemented, or something
like that).  For example, the buf_size argument in this series does
indeed support a non-power-of-two size.

Requesting a granularity to be a power-of-two shouldn't be surprising to
anybody who has an idea of what a bit shift is and how it is used...


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