[PATCH 2/3] cxl: Introduce afu_desc sysfs attribute

[Vaibhav Jain]

Thanks for reviewing the patch Fred!!

Frederic Barrat <fbarrat at linux.vnet.ibm.com> writes:
>> +
>> +	if (afu->attr_afud.size > 0) {
>> +		sysfs_attr_init(&afu->attr_afud.attr);
>> +		afu->attr_afud.attr.name = "afu_desc";
>> +		afu->attr_afud.attr.mode = 0444;
>
>
> So I had a long pause here, wondering if we are showing too much 
> information to ANY user. Most of the content of the AFU descriptor is 
> already world-readable through other sysfs attributes, the only ones 
> troubling me are offset 0x50 and 0x58, ie. the page resolution interrupt 
> handling. We currently don't support it (yet), but we need to think 
> about it. A user process, by monitoring offset x50 could get a glimpse 
> of the layout of the address space of other processes. Not the actual 
> content, but at least what addresses are valid. I think that's already 
> too much.
As we discussed "Paged Resolution EA (offset 0x58)" register is write
only should ideally return 0x00  for all reads (need to confirm with
h/w on this). Secondly there isnt much information exposed from register
"Paged Resolution Handle(offset 0x50)" apart from process handle. Lastly
presently Paged-Response Resolution isn't supported for cxl on linux. So,
I think we are safe for the time being.

> Why not just exporting an area only covering 64 bits (and guaranteed not 
> to be all 1's) and call it mmio_check? It would seem safer to me.
AFAIK the minimum granuality that we have is PAGE_SIZE and we cannot mmap
anything less than PAGE_SIZE. Though one exception for hashed paged
table on power, we have an ability to map 4K PFN on 64K hosts but it
isnt supported for radix on P9.

At best I can limit access to first page of the afu descriptor.


-- 
Vaibhav Jain <vaibhav at linux.vnet.ibm.com>
Linux Technology Center, IBM India Pvt. Ltd.