On 1/15/24 01:09, Borislav Petkov wrote: > On Fri, Jan 12, 2024 at 12:00:01PM -0800, Dave Hansen wrote: >> I thought we agreed long ago to just demote the whole direct map to 4k >> on kernels that might need to act as SEV-SNP hosts. That should be step >> one and this can be discussed as an optimization later. > > Do we have a link to that agreement somewhere? I seem to be remembering it wrong. I have some memory of disabling 2M mappings in the same spots that debug_pagealloc_enabled() does, but I must have hallucinated it. > https://lore.kernel.org/lkml/535400b4-0593-a7ca-1548-532ee1fefbd7@xxxxxxxxx/ The original approach tried to fix up the page faults by doing the demotion there and didn't seem to work with hugetlbfs at all. > I'd like to read why we agreed. And looking at Mike's talk: > https://lwn.net/Articles/931406/ - what do we wanna do in general with > the direct map granularity? I think fracturing it is much less of a problem than we once thought it was. There are _definitely_ people that will pay the cost for added security. The problem will be the first time someone sees a regression when their direct-map-fracturing kernel feature (secret pages, SNP host, etc...) collides with some workload that's sensitive to kernel TLB pressure.
