linux-kernel - Re: [PATCH Part2 v6 07/49] x86/sev: Invalid pages from direct map when adding it to RMP table

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Message-ID: <Yuu3ZK+/hL+saV27@zn.tnic>
Date:   Thu, 4 Aug 2022 14:11:16 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Kalra, Ashish" <Ashish.Kalra@....com>
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Subject: Re: [PATCH Part2 v6 07/49] x86/sev: Invalid pages from direct map
 when adding it to RMP table

On Mon, Aug 01, 2022 at 11:57:09PM +0000, Kalra, Ashish wrote:
> You mean set_memory_present() ?

Right, that.

We have set_memory_np() but set_memory_present(). Talk about
consistence... ;-\

> But again, calling set_direct_map_invalid_noflush() is easier to
> understand from the calling function's point of view as it correlates
> to the functionality of invalidating the page from kernel direct map ?

You mean, we prefer easy to understand to performance?

set_direct_map_invalid_noflush() means crap to me. I have to go look it
up - set memory P or NP is much clearer.

The patch which added those things you consider easier to understand is:

commit d253ca0c3865a8d9a8c01143cf20425e0be4d0ce
Author: Rick Edgecombe <rick.p.edgecombe@...el.com>
Date:   Thu Apr 25 17:11:34 2019 -0700

    x86/mm/cpa: Add set_direct_map_*() functions
    
    Add two new functions set_direct_map_default_noflush() and
    set_direct_map_invalid_noflush() for setting the direct map alias for the
    page to its default valid permissions and to an invalid state that cannot
    be cached in a TLB, respectively. These functions do not flush the TLB.

I don't see how this fits with your use case...

Also, your helpers are called restore_direct_map and
invalidate_direct_map. That's already explaining what this is supposed
to do.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette