[Qemu-devel] Re: [PATCH 1/4] Add a scatter-gather list type and accessors

[Avi Kivity <avi@redhat.com>]

Anthony Liguori wrote:
> Avi Kivity wrote:
>> Scatter-gather lists are used extensively in dma-capable devices; a
>> single data structure allows more code reuse later on.
>>
>> diff --git a/dma-helpers.c b/dma-helpers.c
>> new file mode 100644
>> index 0000000..315834e
>> --- /dev/null
>> +++ b/dma-helpers.c
>> @@ -0,0 +1,29 @@
>>   
>
> Needs copyright/license.

Sure.

Is it possible to have a blanket license for files which don't have 
explicit terms?  I don't much like boilerplate.

>
>> +#include "dma.h"
>> +
>> +
>> +void qemu_sglist_init(QEMUSGList *qsg, int alloc_hint)
>> +{
>> +    qsg->sg = qemu_malloc(alloc_hint * sizeof(ScatterGatherEntry));
>>   
>
> Would be nice to check for malloc failures and fail gracefully at least.

Do you mean an exit(1)?  If so we could just put it in qemu_malloc().

Propagation is usually not possible since hardware is not expected to 
run out of memory.  It will also be a never-tested code path.

>>
>> +void qemu_sglist_add(QEMUSGList *qsg, target_phys_addr_t base,
>> +                     target_phys_addr_t len)
>> +{
>> +    if (qsg->nsg == qsg->nalloc) {
>> +        qsg->nalloc = 2 * qsg->nalloc + 1;
>>   
>
> Do you really want exponential growth verses linear growth?  The sg 
> lists should be relatively small so linear growth should be fine.
>

I expect this to trigger rarely since the allocation hint should suffice 
nearly 100% of the time.  But in case we miss, it's better to reallocate 
as little as possible.

(what I really want is std::vector<>)

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.