Messages in this thread | | | Subject | Re: [PATCH v5 3/3] x86/tdx: Add Quote generation support | From | Kai Huang <> | 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
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