linux-kernel - Re: [PATCH v8 4/5] x86/mm: Add noalias variants of set_memory

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Message-ID: <20220705152910.rq2ga25gnw7kuvt5@box.shutemov.name>
Date:   Tue, 5 Jul 2022 18:29:10 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Kai Huang <kai.huang@...el.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>, Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Wander Lairson Costa <wander@...hat.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>,
        marcelo.cerri@...onical.com, tim.gardner@...onical.com,
        khalid.elmously@...onical.com, philip.cox@...onical.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 4/5] x86/mm: Add noalias variants of
 set_memory_*crypted() functions

On Tue, Jun 28, 2022 at 01:15:21PM +1200, Kai Huang wrote:
> 
> > 
> > I still don't like the idea of using the DMA API itself.  But, maybe we
> > need some common infrastructure that the DMA API and this code use which
> > says, "get me some pages that I can safely make shared".
> 
> Right.  For instance any KVM PV feature would require shared memory, and DMA API
> normally doesn't fit (taking 'struct kvm_steal_time' as example).
> 
> Maybe we can reserve a CMA for this purpose.

CMA for couple low traffic users sounds like an overkill. It will create
an separate pool just for them.

I think the best way is to add an dummy device and couple of helpers
around DMA API that would allow to tap into swiotlb.

Maybe hide it inside CC infrastructure. Like cc_decrypted_alloc() and
cc_decrypted_free().

-- 
 Kirill A. Shutemov