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Re: [Xen-devel] [PATCH 4/6] replace vCPU's dirty CPU mask by numeric ID

>>> On 19.01.18 at 18:48, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 19/01/18 16:06, Jan Beulich wrote:
>> At most one bit can be set in the masks, so especially on larger systems
>> it's quite a bit of unnecessary memory and processing overhead to track
>> the information as a mask. Store the numeric ID of the respective CPU
>> instead, or NR_CPUS if no dirty state exists.
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Definitely +1 for this change.
> However, the comparison against nr_cpu_ids isn't completely obvious as
> to its function.  How about introducing a predicate such as vcpu_dirty()
> which wraps the use of the id?

I can do that.

> Also, you'd get better code by using NR_CPUS which is a compile-time
> constant, rather than nr_cpu_ids which would be a memory read.

I did consider it, but decided that the check being more tight when
using nr_cpu_ids is preferable. The checks sit in ASSERT()s only
anyway, i.e. I don't think performance matters much.


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