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

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Message-ID: <YKK4AHhfv1nrYiw2@google.com>
Date:   Mon, 17 May 2021 18:37:52 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Andi Kleen <ak@...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 Mon, May 17, 2021, Dave Hansen wrote:
> On 5/17/21 11:27 AM, Kuppuswamy, Sathyanarayanan wrote:
> > On 5/17/21 11:16 AM, Sean Christopherson wrote:
> >> What generic code needs access to SEV vs. TDX? 
> >> force_dma_unencrypted() is called from generic code, but its
> >> implementation is x86 specific.
> > 
> > When the hardening the drivers for TDX usage, we will have
> > requirement to check for is_protected_guest() to add code specific to
> > protected guests. Since this will be outside arch/x86, we need common
> > framework for it.
> 
> Just remember, a "common framework" doesn't mean that it can't be backed
> by extremely arch-specific mechanisms.
> 
> For instance, there's a lot of pkey-specific code in mm/mprotect.c.  It
> still gets optimized away on x86 with all the goodness of X86_FEATUREs.

Ya, exactly.  Ideally, generic code shouldn't have to differentiate between SEV,
SEV-ES, SEV-SNP, TDX, etc..., a vanilla "bool is_protected_guest(void)" should
suffice.  Under the hood, x86's implementation for is_protected_guest() can be
boot_cpu_has() checks (if we want).