Re: [PATCH 1/3] wifi: ath11k: fix dest ring-buffer corruption

[Miaoqing Pan <quic_miaoqing@quicinc.com>]

On 6/3/2025 7:51 PM, Johan Hovold wrote:
> On Tue, Jun 03, 2025 at 06:52:37PM +0800, Baochen Qiang wrote:
>> On 6/2/2025 4:03 PM, Johan Hovold wrote:
> 
>>> No, the barrier is needed between reading the head pointer and accessing
>>> descriptor fields, that's what matters.
>>>
>>> You can still end up with reading stale descriptor data even when
>>> ath11k_hal_srng_dst_get_next_entry() returns non-NULL due to speculation
>>> (that's what happens on the X13s).
>>
>> The fact is that a dma_rmb() does not even prevent speculation, no matter where it is
>> placed, right?
> 
> It prevents the speculated load from being used.
> 
>> If so the whole point of dma_rmb() is to prevent from compiler reordering
>> or CPU reordering, but is it really possible?
>>
>> The sequence is
>>
>> 	1# reading HP
>> 		srng->u.dst_ring.cached_hp = READ_ONCE(*srng->u.dst_ring.hp_addr);
>>
>> 	2# validate HP
>> 		if (srng->u.dst_ring.tp == srng->u.dst_ring.cached_hp)
>> 			return NULL;
>>
>> 	3# get desc
>> 		desc = srng->ring_base_vaddr + srng->u.dst_ring.tp;
>>
>> 	4# accessing desc
>> 		ath11k_hal_desc_reo_parse_err(... desc, ...)
>>
>> Clearly each step depends on the results of previous steps. In this case the compiler/CPU
>> is expected to be smart enough to not do any reordering, isn't it?
> 
> Steps 3 and 4 can be done speculatively before the load in step 1 is
> complete as long as the result is discarded if it turns out not to be
> needed.
> 

If the condition in step 2 is true and step 3 speculatively loads 
descriptor from TP before step 1, could this cause issues?

We previously had extensive discussions on this topic in the 
https://lore.kernel.org/linux-wireless/ecfe850c-b263-4bee-b888-c34178e690fc@quicinc.com/ 
thread. On my platform, dma_rmb() did not work as expected. The issue 
only disappeared after disabling PCIe endpoint relaxed ordering in 
firmware side. So it seems that HP was updated (Memory write) before 
descriptor (Memory write), which led to the problem.