Re: [PATCH v5 3/3] x86/tdx: Add Quote generation support

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Kai Huang <kai.huang@intel.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Dave Hansen <dave.hansen@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
Date: Mon, 9 May 2022 15:09:27 +0300	[thread overview]
Message-ID: <20220509120927.7rg6v5pyc3f4pxsh@box.shutemov.name> (raw)
In-Reply-To: <45d184273f1950320843f6696eb3071f7d354fd3.camel@intel.com>

On Mon, May 09, 2022 at 03:37:22PM +1200, Kai Huang wrote:
> On Sat, 2022-05-07 at 03:42 +0300, Kirill A. Shutemov wrote:
> > On Fri, May 06, 2022 at 12:11:03PM +1200, Kai Huang wrote:
> > > Kirill, what's your opinion?
> > 
> > I said before that I think DMA API is the right tool here.
> > 
> > Speculation about future of DMA in TDX is irrelevant here. If semantics
> > change we will need to re-evaluate all users. VirtIO uses DMA API and it
> > is conceptually the same use-case: communicate with the host.
> 
> Virtio is designed for device driver to use, so it's fine to use DMA API. And
> real DMA can happen to the virtio DMA buffers.  Attestation doesn't have such
> assumption.

Whether attestation driver uses struct device is implementation detail.
I don't see what is you point.

> DMA API has it's limitations.

Could you elaborate here?

> So I don't see why TD guest kernel cannot have a simple protocol to vmap() a
> page (or couple of pages) as shared on-demand, like below:
> 
> 	page = alloc_page();
> 
> 	addr = vmap(page,  pgprot_decrypted(PAGE_KERNEL));
> 
> 	clflush_cache_range(page_address(page), PAGE_SIZE);
> 
> 	MapGPA(page_to_phys(page) | cc_mkdec(0), PAGE_SIZE);
> 
> And we can even avoid above clflush_cache_range() if I understand correctly.
> 
> Or  I missed something?

For completeness, cover free path too. Are you going to opencode page
accept too?

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 A. Shutemov