Re: [PATCH v16 13/51] KVM: x86: Enable guest SSP read/write interface with new uAPIs

[Xiaoyao Li]

On 9/20/2025 6:32 AM, Sean Christopherson wrote:
> From: Yang Weijiang<weijiang.yang@xxxxxxxxx>
>
> Add a KVM-defined ONE_REG register, KVM_REG_GUEST_SSP, to let userspace
> save and restore the guest's Shadow Stack Pointer (SSP).  On both Intel
> and AMD, SSP is a hardware register that can only be accessed by software
> via dedicated ISA (e.g. RDSSP) or via VMCS/VMCB fields (used by hardware
> to context switch SSP at entry/exit).  As a result, SSP doesn't fit in
> any of KVM's existing interfaces for saving/restoring state.
>
> Internally, treat SSP as a fake/synthetic MSR, as the semantics of writes
> to SSP follow that of several other Shadow Stack MSRs, e.g. the PLx_SSP
> MSRs.  Use a translation layer to hide the KVM-internal MSR index so that
> the arbitrary index doesn't become ABI, e.g. so that KVM can rework its
> implementation as needed, so long as the ONE_REG ABI is maintained.
>
> Explicitly reject accesses to SSP if the vCPU doesn't have Shadow Stack
> support to avoid running afoul of ignore_msrs, which unfortunately applies
> to host-initiated accesses (which is a discussion for another day).  I.e.
> ensure consistent behavior for KVM-defined registers irrespective of
> ignore_msrs.
>
> Link:https://lore.kernel.org/all/aca9d389-f11e-4811-90cf-d98e345a5cc2@xxxxxxxxx
> Suggested-by: Sean Christopherson<seanjc@xxxxxxxxxx>
> Signed-off-by: Yang Weijiang<weijiang.yang@xxxxxxxxx>
> Tested-by: Mathias Krause<minipli@xxxxxxxxxxxxxx>
> Tested-by: John Allen<john.allen@xxxxxxx>
> Tested-by: Rick Edgecombe<rick.p.edgecombe@xxxxxxxxx>
> Signed-off-by: Chao Gao<chao.gao@xxxxxxxxx>
> Co-developed-by: Sean Christopherson<seanjc@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson<seanjc@xxxxxxxxxx>

Reviewed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>