CVM: refactor ohcl/ovmm emulator for flexible register retrieval and update

[babayet2]

Motivation

1. When an instruction is emulated for a CVM guest, the full guest register state is retrieved. For TDX, this dispatches an expensive TDCALL to read each segment register regardless of whether it is necessary for the emulation.
2. The existing emulator code allowed arbitrary guest registers to be written: if the register is considered immutable, the update is silently dropped.

Changes

• The emulator frontend no longer has a guest state struct. Instead, the frontend calls functions to read and update register state.
• Each backend defines and implements its own register state struct. On a register read/write, the backend has the flexibility to choose whether a register should be read from this struct, or pass through directly to the backing. With the exception of segment registers for the TDX backing, the existing behavior has been preserved, e.g. reading XMM registers passes through to the backing, and GPs are read from the state struct.

[smmalis37]

Take a look at #478 too, lets make sure we don't conflict too hard.

[smalis-msft]

Overall this looks really nice, and a great step in the right direction. Thanks!

[babayet2]

@smalis-msft rebase done. could you kick off another CI run to make sure we're green?

we're trying to get this in before the freeze, do you expect we'll need another reviewer? quick reminder, we cannot test ARM/AMD, so we'll need help on the MSFT side for that validation

[smalis-msft]

CI started, a little bit more feedback posted, and I'll get started on SNP testing. We don't need another reviewer.

[smalis-msft]

Also note that in order for this to get into 1.5 it will need to be cherry-picked to the release/2411 branch after merging, which may involve more merge conflicts.

[babayet2]

@smalis-msft really appreciate all of the detailed and speedy reviews, they have been incredibly helpful

I think we should pass all CI gates after another run is kicked off, and tdx cvms are booting. anything else I should be doing to get this merged?

[smalis-msft]

LGTM! I still need to some more testing but if everything looks good on my end I'll merge it.

[smalis-msft]

Tested on SNP, everything looks good.

[smalis-msft]

Given that this change is focused solely on perf, and not functionality, I'm not sure it makes sense to deal with taking such a large change to 1.5. If it's really desired though you'll need to send up a PR with it cherry-picked onto release/2411. I can test SNP on that as well.