On Mon, Jun 27, 2022 at 01:54:45PM +0200, Ard Biesheuvel wrote: > On Mon, 27 Jun 2022 at 13:30, Kirill A. Shutemov > <kirill.shutemov@xxxxxxxxxxxxxxx> wrote: > > > > On Fri, Jun 24, 2022 at 10:37:10AM -0600, Peter Gonda wrote: > > > On Tue, Jun 14, 2022 at 6:03 AM Kirill A. Shutemov > > > <kirill.shutemov@xxxxxxxxxxxxxxx> wrote: > > > > > > > > UEFI Specification version 2.9 introduces the concept of memory > > > > acceptance: some Virtual Machine platforms, such as Intel TDX or AMD > > > > SEV-SNP, requiring memory to be accepted before it can be used by the > > > > guest. Accepting happens via a protocol specific for the Virtual > > > > Machine platform. > > > > > > > > Accepting memory is costly and it makes VMM allocate memory for the > > > > accepted guest physical address range. It's better to postpone memory > > > > acceptance until memory is needed. It lowers boot time and reduces > > > > memory overhead. > > > > > > > > The kernel needs to know what memory has been accepted. Firmware > > > > communicates this information via memory map: a new memory type -- > > > > EFI_UNACCEPTED_MEMORY -- indicates such memory. > > > > > > > > Range-based tracking works fine for firmware, but it gets bulky for > > > > the kernel: e820 has to be modified on every page acceptance. It leads > > > > to table fragmentation, but there's a limited number of entries in the > > > > e820 table > > > > > > > > Another option is to mark such memory as usable in e820 and track if the > > > > range has been accepted in a bitmap. One bit in the bitmap represents > > > > 2MiB in the address space: one 4k page is enough to track 64GiB or > > > > physical address space. > > > > > > > > In the worst-case scenario -- a huge hole in the middle of the > > > > address space -- It needs 256MiB to handle 4PiB of the address > > > > space. > > > > > > > > Any unaccepted memory that is not aligned to 2M gets accepted upfront. > > > > > > > > The approach lowers boot time substantially. Boot to shell is ~2.5x > > > > faster for 4G TDX VM and ~4x faster for 64G. > > > > > > > > TDX-specific code isolated from the core of unaccepted memory support. It > > > > supposed to help to plug-in different implementation of unaccepted memory > > > > such as SEV-SNP. > > > > > > > > The tree can be found here: > > > > > > > > https://github.com/intel/tdx.git guest-unaccepted-memory > > > > > > Hi Kirill, > > > > > > I have a couple questions about this feature mainly about how cloud > > > customers can use this, I assume since this is a confidential compute > > > feature a large number of the users of these patches will be cloud > > > customers using TDX and SNP. One issue I see with these patches is how > > > do we as a cloud provider know whether a customer's linux image > > > supports this feature, if the image doesn't have these patches UEFI > > > needs to fully validate the memory, if the image does we can use this > > > new protocol. In GCE we supply our VMs with a version of the EDK2 FW > > > and the customer doesn't input into which UEFI we run, as far as I can > > > tell from the Azure SNP VM documentation it seems very similar. We > > > need to somehow tell our UEFI in the VM what to do based on the image. > > > The current way I can see to solve this issue would be to have our > > > customers give us metadata about their VM's image but this seems kinda > > > burdensome on our customers (I assume we'll have more features which > > > both UEFI and kernel need to both support inorder to be turned on like > > > this one) and error-prone, if a customer incorrectly labels their > > > image it may fail to boot.. Has there been any discussion about how to > > > solve this? My naive thoughts were what if UEFI and Kernel had some > > > sort of feature negotiation. Maybe that could happen via an extension > > > to exit boot services or a UEFI runtime driver, I'm not sure what's > > > best here just some ideas. > > > > Just as an idea, we can put info into UTS_VERSION which can be read from > > the built bzImage. We have info on SMP and preeption there already. > > > > Instead of hacking this into the binary, couldn't we define a protocol > that the kernel will call from the EFI stub (before EBS()) to identify > itself as an image that understands unaccepted memory, and knows how > to deal with it? > > That way, the firmware can accept all the memory on behalf of the OS > at ExitBootServices() time, unless the OS has indicated there is no > need to do so. I agree it would be better. But I think it would require change to EFI spec, no? -- Kirill A. Shutemov
