linux-kernel - Re: [PATCH 00/16] mm: Introduce MAP_BELOW

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Message-ID: <4219f619-4b32-40bc-85b8-cb11d76fde98@intel.com>
Date: Thu, 29 Aug 2024 09:54:08 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Charlie Jenkins <charlie@...osinc.com>
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Subject: Re: [PATCH 00/16] mm: Introduce MAP_BELOW_HINT

On 8/28/24 13:15, Charlie Jenkins wrote:
> A way to restrict mmap() to return LAM compliant addresses in an entire
> address space also doesn't have to be mutually exclusive with this flag.
> This flag allows for the greatest degree of control from applications.
> I don't believe there is additionally performance saving that could be
> achieved by having this be on a per address space basis.

I agree with you in general.  The MAP_BELOW_HINT _is_ the most flexible.
 But it's also rather complicated.

My _hope_ would be that a per-address-space property could share at
least some infrastructure with what x86/LAM and arm/TBI do to the
address space.  Basically put the restrictions in place for purely
software reasons instead of the mostly hardware reasons for LAM/TBI.

Lorenzo also raised some very valid points about a having a generic
address-restriction ABI.  I'm certainly not discounting those concerns.
It's not something that can be done lightly.