Re: [RFC PATCH v2 20/44] i386/tdx: Parse tdx metadata and store the result into TdxGuestState

[Xiaoyao Li]

On 1/25/2022 3:42 PM, Gerd Hoffmann wrote:
> > Regarding what interface should be used to load TDVF, there are three
> > options:
> >
> >    1) pflash: the same as how we load OVMF.
> >
> >    Suppose TDVF support will finally get into OVMF, using this
> >    interface, it's full compatible with normal VMs. No change required
> >    to QEMU command line and TDVF binary is mapped to the GPA address
> >    right below 4G, but they are actually mapped as RAM.
> >
> >    Of course, we need several hacks (special handling) in QEMU.
>
> What kind if "hack"?

For example, several pflash codes require the support of read-only 
memslot from KVM. We need to absolve it for TDX guest.

And the pflash won't work as a flash device that no write induced 
KVM_EXIT_MMIO will go to its callback.

> >    Of course, they don't work as flash, the change made to it doesn't
> >    persist.
> >
> >    2) -bios
> >
> >    exploit "-bios" parameter to load TDVF. But again, read-only is not
> >    supported. TDVF is mapped as RAM.
> >
> >    3) generic loader
> >
> >    Just like what this series does. Implement specific parser in generic
> >    loader to load and parse TDVF as private RAM.
> >
> > I'm nor sure if 1) is acceptable from your side. If not, we will go with
> > option 3, since 2) doesn't make too much sense.
>
> Yep, Daniel (Cc'ed) tried (2) recently for SEV.  Didn't work due to
> differences in -bios and -pflash reset handling.  So that option is
> out I guess.
>
> SEV uses (1), and there is some support code which you should be able to
> reuse (walker for the list of guid-sections with meta-data in the ovmf
> reset vector for example).
>
> So TDX going for (1) too is probably the best option.

Yes. With option 1), it's friendly to user that no command change.
And also compatible for future TDX/TDVF when non-volatile variable is 
supported.

Next version of this series will go with option 1). Let's wait and see 
if the real implementation is acceptable or not.

> take care,
>    Gerd