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

[Michael Roth <michael.roth@amd.com>]

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.

-Mike