Re: [PATCH v1 11/26] x86/sev: Invalidate pages from the direct map when adding them to the RMP table

Dave Hansen
2024-01-16T16:21:16Z

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@intel.com/

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.