Re: [PATCH] memory: transaction API

Ferry Huberts
2011-07-21T12:26:05Z

On 07/21/2011 02:07 PM, Avi Kivity wrote:
> On 07/21/2011 02:04 PM, Ferry Huberts wrote:
>> On 07/21/2011 12:21 PM, Avi Kivity wrote:
>> > Allow changes to the memory hierarchy to be accumulated and
>> > made visible all at once. This reduces computational effort,
>> > especially when an accelerator (e.g. kvm) is involved.
>> >
>> > Useful when a single register update causes multiple changes
>> > to an address space.
>> >
>> >
>> > +void memory_region_transaction_begin(void)
>> > +{
>> > + ++memory_region_transaction_depth;
>> > +}
>> > +
>>
>> wouldn't you rather keep it safe by doing either here
>>
>> if (!memory_region_transaction_depth)
>> memory_region_transaction_depth++;
>>
>
> Why? I want to allow nesting transactions (not that I anticipate such a
> case).
>
>> > +void memory_region_transaction_commit(void)
>> > +{
>> > + if (!memory_region_transaction_depth) {
>> > + abort();
>> > + }
>>
>>
>> > + --memory_region_transaction_depth;
>> > + memory_region_update_topology();
>> > +}
>> > +
>>
>> or by doing here
>>
>> while (!memory_region_transaction_depth)
>> memory_region_transaction_depth--;

doesn't memory_region_update_topology commit all accumulated changes? if
it does then memory_region_transaction_depth is left non-zero in the
nesting case while no more changes are actually present, resulting in
superfluous calls to memory_region_update_topology.

maybe I misunderstood memory_region_update_topology?

>>
>>
>> with your setup nesting transactions will not work correctly I think.
>> You seem to have designed it to not do nesting, so it's safer to make
>> that explicit in your code imho
>
> Nesting should work just fine.
>
>> therefore I'd go for the change in _begin
>>
>> also, wouldn't you rather rename memory_region_transaction_depth to
>> memory_region_transaction_pending?
>
> The existing name works for me, but if people want it changed, that's
> fine too.
>
>

--
Ferry Huberts