linux-kernel - Re: [RFC v2 26/32] x86/mm: Move force_dma

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Date:   Wed, 12 May 2021 16:08:21 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 26/32] x86/mm: Move force_dma_unencrypted() to common
 code

On Mon, May 10, 2021 at 03:23:29PM -0700, Dave Hansen wrote:
> On 5/10/21 3:19 PM, Kuppuswamy, Sathyanarayanan wrote:
> > On 5/7/21 2:54 PM, Dave Hansen wrote:
> >> This doesn't seem much like common code to me.  It seems like 100% SEV
> >> code.  Is this really where we want to move it?
> > 
> > Both SEV and TDX code has requirement to enable
> > CONFIG_ARCH_HAS_FORCE_DMA_UNENCRYPTED and define force_dma_unencrypted()
> > function.
> > 
> > force_dma_unencrypted() is modified by patch titled "x86/tdx: Make DMA
> > pages shared" to add TDX guest specific support.
> > 
> > Since both SEV and TDX code uses it, its moved to common file.
> 
> That's not an excuse to have a bunch of AMD (or Intel) feature-specific
> code in a file named "common".  I'd make an attempt to keep them
> separate and then call into the two separate functions *from* the common
> function.

But why? What good does the additional level of inderection brings?

It's like saying arch/x86/kernel/cpu/common.c shouldn't have anything AMD
or Intel specific. If a function can cover both vendors I don't see a
point for additinal complexity.

-- 
 Kirill A. Shutemov