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

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Message-ID: <YKKzCOW9u6q06E5I@google.com>
Date:   Mon, 17 May 2021 18:16:40 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "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 Thu, May 13, 2021, 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.

What generic code needs access to SEV vs. TDX?  force_dma_unencrypted() is called
from generic code, but its implementation is x86 specific.

> > 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

SEV already has the sev_enable_key static key that it uses for unrolling string
I/O, so there's at least one (debatable) case that wants to use static branches.

For SEV-ES and TDX, there's a better argument as using X86_FEATURE_* would unlock
alternatives.

> BTW right now I'm not even sure we need the bitmap for anything, but I guess
> it doesn't hurt.
> 
> -Andi
> 
>