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Date:   Mon, 9 May 2022 08:43:03 -0700
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Subject: Re: [PATCH v5 3/3] x86/tdx: Add Quote generation support
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To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Dave Hansen <dave.hansen@intel.com>
Cc:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        Kai Huang <kai.huang@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
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From:   Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
In-Reply-To: <20220509153534.i6v4xcb2g24ia2i4@black.fi.intel.com>
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On 5/9/22 8:35 AM, Kirill A. Shutemov wrote:
> 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.

Following is the link for the SEV attestation driver. It does seem to
use alloc_pages() and set_memory_*() calls.

https://lore.kernel.org/lkml/20220307215344.2799259-1-brijesh.singh@amd.com/

> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer