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

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Date:   Thu, 13 May 2021 12:42:28 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andi Kleen <ak@...ux.intel.com>,
        "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Raj Ashok <ashok.raj@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 26/32] x86/mm: Move force_dma_unencrypted() to common
 code

On 5/13/21 12:38 PM, Andi Kleen wrote:
> 
> On 5/13/2021 10:49 AM, Dave Hansen wrote:
>> On 5/13/21 9:40 AM, Kuppuswamy, Sathyanarayanan wrote:
>>> +#define PROTECTED_GUEST_BITMAP_LEN    128
>>> +
>>> +/* Protected Guest vendor types */
>>> +#define GUEST_TYPE_TDX            (1)
>>> +#define GUEST_TYPE_SEV            (2)
>>> +
>>> +/* Protected Guest features */
>>> +#define MEMORY_ENCRYPTION        (20)
>> I was assuming we'd reuse the X86_FEATURE infrastructure somehow.  Is
>> there a good reason not to?
> 
> This for generic code. Would be a gigantic lift and lots of refactoring
> to move that out.

Ahh, forgot about that.  The whole "x86/mm" subject threw me off.

>> That gives us all the compile-time optimization (via
>> en/disabled-features.h) and static branches for "free".
> 
> There's no user so far which is anywhere near performance critical, so
> that would be total overkil

The *REALLY* nice thing is that it keeps you from having to create stub
functions or #ifdefs and yet the compiler can still optimize the code to
nothing.

Anyway, thanks for the clarification about it being in non-arch code.