Messages in this thread | | | Date | Fri, 22 Jul 2022 21:18:44 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCHv7 02/14] mm: Add support for unaccepted memory |
| |
On Thu, Jul 21, 2022 at 08:49:31AM -0700, Dave Hansen wrote: > Acceptance is slow and the heavy lifting is done inside the TDX module. > It involves flushing old aliases out of the caches and initializing the > memory integrity metadata for every cacheline. This implementation does > acceptance in 2MB chunks while holding a global lock.
Oh, fun.
> So, those (effective) 2MB clflush+memset's (plus a few thousand cycles > for the hypercall/tdcall transitions)
So this sounds strange - page validation on AMD - judging by the pseudocode of the PVALIDATE insn - does a bunch of sanity checks on the gVA of the page and then installs it into the RMP and also "PVALIDATE performs the same segmentation and paging checks as a 1-byte read. PVALIDATE does not invalidate TLB caches."
But that still sounds a lot less work than what the TDX module needs to do...
> can't happen in parallel. They are serialized and must wait on each > other.
Ofc, the Intel version of the RMP table needs to be protected. :-)
> If you have a few hundred CPUs all trying to allocate memory (say, > doing the first kernel compile after a reboot), this is going to be > very, very painful for a while. > > That said, I think this is the right place to _start_. There is going > to need to be some kind of follow-on solution (likely background > acceptance of some kind). But, even with that solution, *this* code > is still needed to handle the degenerate case where the background > accepter can't keep up with foreground memory needs.
I'm still catering to the view that it should be a two-tier thing: you validate during boot a certain amount - say 4G - a size for which the boot delay is acceptable and you do the rest on-demand along with a background accepter.
That should give you the best of both worlds...
Thx.
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
| |