On 6/5/2015 10:12 AM, Borislav Petkov wrote:
On Fri, Jun 05, 2015 at 10:05:13AM -0700, Zhang, Jonathan Zhixiong wrote:
What is DDR?
I think this needs to be clarified first before we go any further.
I thought the word "memory" might be confusing, because there are
So you mean normal RAM here?
Yes, exactly. I should use this word RAM instead.
memories on the system that is not accessible by Linux. In this context, the APEI error data is accessed (read and write) by both Linux and platform firmware; hence both sides should access the memory using same cache attribute. I wanted to emphasize the idea that even though normally DDR is cachable, but in this case when platform access it with un-cached attribute, Linux should do the same.
Makes sense.
Btw, do we need synchronization between firmware and Linux then? Does Linux need to know when it is ok to touch that memory?
Good question. Linux zeros out error status code in the error data after the data is consumed, this is good; but it alone does not solve the synchronization concern.
For interrupt notification type (SCI or NMI) error source, this may not be an issue since both sides can operate under the rule that the error data is only overwritten but never appended. But what about poll notification type? In this case, platform gathers error, updates the memory region as needed; Linux checks the same memory region periodically.
An ACPI APEI proposal intended to solve this concern has been discussed in UEFI forum. The idea is to have OS to send platform a signal (through updating a designated register) after error data is consumed. Therefore, when OS is accessing the memory region, platform does not try to access the same memory region in the mean time.
After this proposal is approved and published, I will submit a patch to implement it.