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Date:   Mon, 9 May 2022 18:35:34 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        Kai Huang <kai.huang@intel.com>,
        Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H . Peter Anvin" <hpa@zytor.com>, Tony Luck <tony.luck@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Wander Lairson Costa <wander@redhat.com>,
        Isaku Yamahata <isaku.yamahata@gmail.com>,
        marcelo.cerri@canonical.com, tim.gardner@canonical.com,
        khalid.elmously@canonical.com, philip.cox@canonical.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 3/3] x86/tdx: Add Quote generation support
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On Mon, May 09, 2022 at 07:14:20AM -0700, Dave Hansen wrote:
> On 5/9/22 05:09, Kirill A. Shutemov wrote:
> > Private->Shared conversion is destructive. You have to split SEPT, flush
> > TLB. Backward conversion even more costly.
> > 
> > Rule of thumb is avoid conversion where possible. DMA API is there for
> > you.
> 
> Kirill, I understand that the DMA API is a quick fix today.  But is it
> _really_ the right long-term interface?

Yes, I think so.

> There will surely come a time when TDX I/O devices won't be using fixed
> bounce buffers.  What will the quote generation code do then?  How will
> we know to come back around and fix this up?

VirtIO will not go away with TDX I/O in picture. TDX I/O will be addition
to existing stuff, not replacement.

And we have hooks in place to accommodate this: force_dma_unencrypted()
will return false for devices capable of TDX I/O. While the rest of
devices, including VirtIO and attestation, keep using existing paths with
swiotlb.

> Does SEV or the s390 ultravisor need anything like this?

At quick glance sev-guest.c uses set_memory_decrypted()/encrypted() for
allocation and freeing shared memory. I consider it inferior to using DMA
API.

-- 
 Kirill A. Shutemov