On 03/05/2018 02:04 PM, Dave Hansen wrote:
On 03/05/2018 12:28 PM, Khalid Aziz wrote:
Do you have a way to tell that data is not being thrown away? Like if
the ADI metadata is different for two different cachelines within a
single page?
Yes, since access to tagged data is made using pointers with ADI tag
embedded in the top bits, any mismatch between what app thinks the ADI
tags should be and what is stored in the RAM for corresponding page will
result in exception. If ADI data gets thrown away, we will get an ADI
tag mismatch exception. If ADI tags for two different ADI blocks on a
page are different when app expected them to be the same, we will see an
exception on access to the block with wrong ADI data.
So, when an app has two different ADI tags on two parts of a page, the
page gets swapped, and the ADI block size is under PAGE_SIZE, the app
will get an ADI exception after swap-in through no fault of its own?
Only if the kernel fails to re-establish ADI tags on the swapped in page
which is why I added infrastructure to save the ADI tags for a page
before it is swapped out and then re-establish those tags when the page
is swapped back in. Kernel needs to save as many as ADI TAGS as may
exist on each page, not just one tag per page. On sparc M7 8K pages,
there are 128 ADI tags for the page, so kernel will store and restore
128 ADI tags for each page on swap-out and swap-in. If kernel restores
only one ADI tag for the page on swap in, app will get an exception and
it will be kernel's fault.
--
Khalid
