On 10/4/22 19:17, Andrew Cooper wrote: > On 29/09/2022 23:29, Rick Edgecombe wrote: >> From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> >> >> There is essentially no room left in the x86 hardware PTEs on some OSes >> (not Linux). That left the hardware architects looking for a way to >> represent a new memory type (shadow stack) within the existing bits. >> They chose to repurpose a lightly-used state: Write=0,Dirty=1. > How does "Some OSes have a greater dependence on software available bits > in PTEs than Linux" sound? > >> The reason it's lightly used is that Dirty=1 is normally set _before_ a >> write. A write with a Write=0 PTE would typically only generate a fault, >> not set Dirty=1. Hardware can (rarely) both set Write=1 *and* generate the >> fault, resulting in a Dirty=0,Write=1 PTE. Hardware which supports shadow >> stacks will no longer exhibit this oddity. > Again, an interesting anecdote but not salient information here. As much as I like the sound of my own voice (and anecdotes), I agree that this is a bit oblique for the patch. Maybe this anecdote should get banished elsewhere. The changelog here could definitely get to the point faster.
