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: <Y68BlcnT0vrpcwf5@kernel.org>
Date:   Fri, 30 Dec 2022 17:19:49 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Michael Roth <michael.roth@....com>,
        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 09:08:31PM +0100, Borislav Petkov wrote:
> 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.

I had a patch that just adds numpages parameter:

https://lore.kernel.org/lkml/20201123095432.5860-4-rppt@kernel.org/

The set_direct_map*() are not too widely used, so it's not a big deal to
update all callers.
 
> 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
> 

-- 
Sincerely yours,
Mike.