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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Date:   Mon, 19 Dec 2022 21:08:31 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Michael Roth <michael.roth@....com>
Cc:     Ashish Kalra <Ashish.Kalra@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...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, dovmurik@...ux.ibm.com, tobin@....com,
        vbabka@...e.cz, kirill@...temov.name, ak@...ux.intel.com,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, alpergun@...gle.com,
        dgilbert@...hat.com, jarkko@...nel.org
Subject: Re: [PATCH Part2 v6 07/49] x86/sev: Invalid pages from direct map
 when adding it to RMP table

On Mon, Dec 19, 2022 at 09:00:26AM -0600, Michael Roth wrote:
> We implemented this approach for v7, but it causes a fairly significant
> performance regression, particularly for the case for npages > 1 which
> this change was meant to optimize.
> 
> I still need to dig in a big but I'm guessing it's related to flushing
> behavior.

Well, AFAICT, change_page_attr_set_clr() flushes once at the end.

Don't you need to flush when you modify the direct map?

> It would however be nice to have a set_direct_map_default_noflush()
> variant that accepted a 'npages' argument, since it would be more
> performant here and also would potentially allow for restoring the 2M
> direct mapping in some cases. Will look into this more for v8.

set_pages_direct_map_default_noflush()

I guess.

Although the name is a mouthful so I wouldn't mind having those
shortened.

In any case, as long as that helper is properly defined and documented,
I don't mind.

Thx.

-- 
Regards/Gruss,
    Boris.

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