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

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Date: Tue, 16 Jan 2024 08:21:09 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Michael Roth <michael.roth@....com>, x86@...nel.org, kvm@...r.kernel.org,
 linux-coco@...ts.linux.dev, linux-mm@...ck.org,
 linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
 tglx@...utronix.de, mingo@...hat.com, jroedel@...e.de,
 thomas.lendacky@....com, hpa@...or.com, ardb@...nel.org,
 pbonzini@...hat.com, seanjc@...gle.com, vkuznets@...hat.com,
 jmattson@...gle.com, luto@...nel.org, dave.hansen@...ux.intel.com,
 slp@...hat.com, pgonda@...gle.com, peterz@...radead.org,
 srinivas.pandruvada@...ux.intel.com, rientjes@...gle.com, tobin@....com,
 vbabka@...e.cz, kirill@...temov.name, ak@...ux.intel.com,
 tony.luck@...el.com, sathyanarayanan.kuppuswamy@...ux.intel.com,
 alpergun@...gle.com, jarkko@...nel.org, ashish.kalra@....com,
 nikunj.dadhania@....com, pankaj.gupta@....com, liam.merwick@...cle.com,
 zhi.a.wang@...el.com, Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v1 11/26] x86/sev: Invalidate pages from the direct map
 when adding them to the RMP table

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.