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Subject: Re: [PATCH v5 3/3] x86/tdx: Add Quote generation support
From:   Kai Huang <kai.huang@intel.com>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
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
Date:   Tue, 10 May 2022 22:42:10 +1200
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On Tue, 2022-05-10 at 11:54 +1200, Kai Huang wrote:
> On Mon, 2022-05-09 at 15:09 +0300, Kirill A. Shutemov wrote:
> > 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.
> 
> No real DMA is involved in attestation.
> 
> > 
> > > 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?
> 
> Call __tdx_module_call(TDX_ACCEPT_PAGE, ...) right after MapGPA() to convert
> back to private.  I don't think there is any problem?
> 
> > 
> > Private->Shared conversion is destructive. You have to split SEPT, flush
> > TLB. Backward conversion even more costly.
> 
> I think I won't call it destructive.
> 
> And I suggested before, we can allocate a default size buffer (i.e. 4 pages),
> which is large enough to cover all requests for now, during driver
> initialization.  This avoids IOCTL time conversion.  We should still have code
> in the IOCTL to check the request buffer size and when it is larger than the
> default, the old should be freed a larger one should be allocated.  But for now
> this code path will never happen.
> 
> Btw above is based on assumption that we don't support concurrent IOCTLs.  This
> version Sathya somehow changed to support concurrent IOCTLs but this was a
> surprise as I thought we somehow agreed we don't need to support this.

Hi Dave,

Sorry I forgot to mention that GHCI 1.5 defines a generic TDVMCALL<Service> for
a TD to communicate with VMM or another TD or some service in the host.  This
TDVMCALL can support many sub-commands.  For now only sub-commands for TD
migration is defined, but we can have more.

For this, we cannot assume the size of the command buffer, and I don't see why
we don't want to support concurrent TDVMCALLs.  So looks from long term, we will
very likely need IOCTL time buffer private-shared conversion.


-- 
Thanks,
-Kai