On Fri, Jun 24, 2022 at 11:41 AM Michael Roth <michael.roth@xxxxxxx> 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://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fintel%2Ftdx.git&data=05%7C01%7Cmichael.roth%40amd.com%7C73bacba017c84291482a08da55ffd481%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637916854542432349%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=P%2FUJOL305xo85NLXGxGouQVGHgzLJpmBdNyZ7Re5%2FB0%3D&reserved=0 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. > > Not sure if you've seen this thread or not, but there's also been some > discussion around this in the context of the UEFI support: > > https://patchew.org/EDK2/cover.1654420875.git.min.m.xu@xxxxxxxxx/cce5ea2aaaeddd9ce9df6fa7ac1ef52976c5c7e6.1654420876.git.min.m.xu@xxxxxxxxx/#20220608061805.vvsjiqt55rqnl3fw@xxxxxxxxxxxxxxxxxxxxxx > > 2 things being discussed there really, which I think roughly boil down > to: > > 1) how to configure OVMF to enable/disable lazy acceptance > - compile time option most likely: accept-all/accept-minimum/accept-1GB > > 2) how to introduce an automatic mode in the future where OVMF does the > right thing based on what the guest supports. Gerd floated the idea of > tying it to ExitBootServices as well, but not sure there's a solid > plan on what to do here yet. > > If that's accurate, it seems like the only 'safe' option is to disable it via > #1 (accept-all), and then when #2 comes along, compile OVMF to just Do The > Right Thing. > > Users who know their VMs implement lazy acceptance can force it on via > accept-all OVMF compile option. Thanks for this Mike! I will bring this to the EDK2 community. The issue for us is our users use a GCE built EDK2 not their own compiled version so they don't have the choice. Reading the Azure docs it seems the same for them, and for AWS so I don't know how often customers actually get to bring their own firmware. > > -Mike
