Re: [PATCHv7 00/14] mm, x86/cc: Implement support for unaccepted memory

[Peter Gonda <pgonda@google.com>]

On Fri, Jun 24, 2022 at 11:41 AM Michael Roth <michael.roth@amd.com> 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@linux.intel.com> 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&amp;data=05%7C01%7Cmichael.roth%40amd.com%7C73bacba017c84291482a08da55ffd481%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637916854542432349%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=P%2FUJOL305xo85NLXGxGouQVGHgzLJpmBdNyZ7Re5%2FB0%3D&amp;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@intel.com/cce5ea2aaaeddd9ce9df6fa7ac1ef52976c5c7e6.1654420876.git.min.m.xu@intel.com/#20220608061805.vvsjiqt55rqnl3fw@sirius.home.kraxel.org
>
> 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