Re: [PATCH v12 15/24] KVM: VMX: Emulate read and write to CET MSRs

[Edgecombe, Rick P]

On Tue, 2025-08-19 at 09:09 -0700, Sean Christopherson wrote:
> This emulation is wrong (in no small part because the architecture sucks).  From
> the SDM:
> 
>   If the processor does not support Intel 64 architecture, these fields have only
>   32 bits; bits 63:32 of the MSRs are reserved.
> 
>   On processors that support Intel 64 architecture this value cannot represent a
>   non-canonical address.
> 
>   In protected mode, only 31:0 are loaded.
> 
> That means KVM needs to drop bits 63:32 if the vCPU doesn't have LM or if the vCPU
> isn't in 64-bit mode.  The last one is especially frustrating, because software
> can still get a 64-bit value into the MSRs while running in protected, e.g. by
> switching to 64-bit mode, doing WRMSRs, then switching back to 32-bit mode.
> 
> But, there's probably no point in actually trying to correctly emulate/virtualize
> the Protected Mode behavior, because the MSRs can be written via XRSTOR, and to
> close that hole KVM would need to trap-and-emulate XRSTOR.  No thanks.
> 
> Unless someone has a better idea, I'm inclined to take an erratum for this, i.e.
> just sweep it under the rug.

Sounds ok to me. All I could think would be something like use the CR/EFER
interceptions and just exit to userspace if (CR0.PE && !EFER.LM && CR4.CET). But
this would require some rototilling and then likely remain un-exercised. Not
sure it's worth it.