On 11/21/24 03:12, Peter Zijlstra wrote: >> I see e.g. ds_clear_cea() clears PTEs that can have the _PAGE_GLOBAL flag, >> and it correctly uses the non-deferrable flush_tlb_kernel_range(). > > I always forget what we use global pages for, dhansen might know, but > let me try and have a look. > > I *think* we only have GLOBAL on kernel text, and that only sometimes. I think you're remembering how _PAGE_GLOBAL gets used when KPTI is in play. Ignoring KPTI for a sec... We use _PAGE_GLOBAL for all kernel mappings. Before PCIDs, global mappings let the kernel TLB entries live across CR3 writes. When PCIDs are in play, global mappings let two different ASIDs share TLB entries. When KPTI is around, the kernel writes CR3 at user/kernel switches to make sure secrets are unmapped and can't be leaked by Meltdown. But unmapping those secrets doesn't do squat if they were mapped globally since they'll still be in the TLB and quite usable. There, we're more judicious and only mark performance-sensitive things that are not secret to be global, like kernel text.
