On Wed, 2022-10-05 at 07:08 -0700, Dave Hansen wrote: > 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. Although this text was inherited, I thought it was useful to disperse any "huh, I wonder why" thoughts that may be lingering in the readers head as they try to grok the rest of the text. I'll shorten it as suggested. Thanks all.
