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Date: Tue, 16 Jan 2024 08:21:09 -0800
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Subject: Re: [PATCH v1 11/26] x86/sev: Invalidate pages from the direct map
 when adding them to the RMP table
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To: Borislav Petkov <bp@alien8.de>
Cc: Michael Roth <michael.roth@amd.com>, x86@kernel.org, kvm@vger.kernel.org,
 linux-coco@lists.linux.dev, linux-mm@kvack.org,
 linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org,
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References: <20231230161954.569267-1-michael.roth@amd.com>
 <20231230161954.569267-12-michael.roth@amd.com>
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From: Dave Hansen <dave.hansen@intel.com>
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In-Reply-To: <20240115090948.GBZaT2XKw00PokD-WJ@fat_crate.local>
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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.