Re: [dpdk-dev] [PATCH v3 1/1] vfio: modify spapr iommu support to use static window sizing

[David Christensen <drc@linux.vnet.ibm.com>]

On 9/17/20 4:13 AM, Burakov, Anatoly wrote:
> On 10-Aug-20 10:07 PM, David Christensen wrote:
>> The SPAPR IOMMU requires that a DMA window size be defined before memory
>> can be mapped for DMA. Current code dynamically modifies the DMA window
>> size in response to every new memory allocation which is potentially
>> dangerous because all existing mappings need to be unmapped/remapped in
>> order to resize the DMA window, leaving hardware holding IOVA addresses
>> that are temporarily unmapped.  The new SPAPR code statically assigns
>> the DMA window size on first use, using the largest physical memory
>> memory address when IOVA=PA and the highest existing memseg virtual
>> address when IOVA=VA.
>>
>> Signed-off-by: David Christensen <drc@linux.vnet.ibm.com>
>> ---
> 
> <snip>
> 
>> +struct spapr_size_walk_param {
>> +    uint64_t max_va;
>> +    uint64_t page_sz;
>> +    int external;
>> +};
>> +
>> +/*
>> + * In order to set the DMA window size required for the SPAPR IOMMU
>> + * we need to walk the existing virtual memory allocations as well as
>> + * find the hugepage size used.
>> + */
>>   static int
>> -vfio_spapr_unmap_walk(const struct rte_memseg_list *msl,
>> -        const struct rte_memseg *ms, void *arg)
>> +vfio_spapr_size_walk(const struct rte_memseg_list *msl, void *arg)
>>   {
>> -    int *vfio_container_fd = arg;
>> +    struct spapr_size_walk_param *param = arg;
>> +    uint64_t max = (uint64_t) msl->base_va + (uint64_t) msl->len;
>> -    /* skip external memory that isn't a heap */
>> -    if (msl->external && !msl->heap)
>> -        return 0;
>> +    if (msl->external) {
>> +        param->external++;
>> +        if (!msl->heap)
>> +            return 0;
>> +    }
> 
> It would be nice to have some comments in the code explaining what we're 
> skipping and why.

Reviewing this again, my inclination is to skip ALL external memory, 
which by definition would seem to be outside of IOMMU control, so the 
code would read:

    if (msl->external) {
        param->external++;
        return 0;
    }

Not sure why existing code such as vfio_spapr_map_walk() distinguishes 
between heap and non-heap in this situation.  Are there instances in x86 
where it would matter?

> Also, seems that you're using param->external as bool? This is a 
> non-public API so using stdbool is not an issue here, perhaps replace it 
> with bool param->has_external?

Why do you think the distinction is necessary?

Dave