On Thu, 2025-01-09 at 13:18 -0800, Dave Hansen wrote: > > But actually I think INVLPGB is *WAY* better than INVLPG here. > INVLPG > doesn't have ranged invalidation. It will only architecturally > invalidate multiple 4K entries when the hardware fractured them in > the > first place. I think we should probably take advantage of what > INVLPGB > can do instead of following the INVLPG approach. > > INVLPGB will invalidate a range no matter where the underlying > entries > came from. Its "increment the virtual address at the 2M boundary" > mode > will invalidate entries of any size. That's my reading of the docs at > least. Is that everyone else's reading too? Ohhhh, good point! I glossed over that the first half dozen times I was reading the document, because I was trying to use the ASID, and working to figure out why things kept crashing (turns out I can only use the PCID on bare metal) > > So, let's pick a number "Z" which is >= invlpgb_count_max. Z could > arguably be set to tlb_single_page_flush_ceiling. Then do this: > > 4k -> Z*4k => use 4k step > >Z*4k -> Z*2M => use 2M step > >Z*2M => invalidate everything > > Invalidations <=Z*4k are exact. They never zap extra TLB entries. > > Invalidations that use the 2M step *might* unnecessarily zap some > extra > 4k mappings in the last 2M, but this is *WAY* better than > invalidating > everything. > This is a great idea. Then the code in get_flush_tlb_info can adjust start, end, and stride_shift as needed. INVLPGB also supports invalidation of an entire 1GB region, so we can take your idea one step further :) With up to 8 pages zapped by a single INVLPGB instruction, and multiple in flight simultaneously, maybe we could set the threshold to 64, for 8 INVLPGBs in flight at once? That way we can invalidate up to 1/8th of a 512 entry range with individual zaps, before just zapping the higher level entry. > "Invalidate everything" obviously stinks, but it should only be for > pretty darn big invalidations. That would only come into play when we get past several GB worth of invalidation. -- All Rights Reversed.
