Re: [edk2-devel] Move X2APIC enablement from Pei to Sec phase

["Ard Biesheuvel via groups.io" <ardb=kernel.org@groups.io>]

Hello Melody,

On Tue, 22 Apr 2025 at 18:44, Melody (Huibo) Wang <huibo.wang@amd.com> wrote:
>
> Hi,
>
> I am currently working on enabling Alternate Injection for AMD SEV-SNP guests and have encountered a design issue.
>
> The Alternate Injection specification, which is still preliminary, defines a so-called SVSM APIC protocol through a subset
> of X2APIC MSRs while timer support is configurable.
> [ This means, if timer functionality is not supported, the guest must rely on the hypervisor to emulate timer
>  support through use of the #HV Timer GHCB protocol. ]
>
> When the OVMF firmware starts, it is in XAPIC mode by default and then, later, during the init phase it switches the guest to X2APIC.
> However, with Alternate Injection enabled, the OVMF in its very first phase - SEC - does XAPIC accesses.
>
> The SVSM, however, which is part of the guest, uses the so-called SVSM APIC protocol which uses a subset of the X2APIC MSRs.
>
> The OVMF, however, assumes it starts off in XAPIC memory-mapped mode and thus there's a protocol mismatch of sorts
> because with Alternate Injection already enabled in the SEC phase, it mandates X2APIC MSR accesses.
>
> The registers (timer registers) when not handled by SVSM will get routed to the hypervisor (KVM) which at that point is operating the guest
> in XAPIC mode until the PEI phase switches to X2APIC.
>
> If X2APIC enablement is moved from the PEI to the SEC phase, the problem can be resolved. I have tested it and it works.
> However, I dont know if there is any concern or potential design issues with that move.
>
> Do folks think this is ok to do - i.e., move the X2APIC enablement to the SEC phase?
>
> Or do you have any suggestions for a better solution?
>

Please implement your proposed changes as a Github PR against the
tianocore/edk2 project. The relevant maintainers will automatically be
cc'ed and we can continue the discussion there.

(Presenting a concrete solution is usually more effective in eliciting
responses, positive or otherwise)


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