Re: [PATCH v15 04/41] KVM: x86: Introduce KVM_{G,S}ET_ONE_REG uAPIs support

[Binbin Wu]

On 9/13/2025 7:22 AM, Sean Christopherson wrote:
> From: Yang Weijiang <weijiang.yang@xxxxxxxxx>
>
> Enable KVM_{G,S}ET_ONE_REG uAPIs so that userspace can access MSRs and
> other non-MSR registers through them, along with support for
> KVM_GET_REG_LIST to enumerate support for KVM-defined registers.
>
> This is in preparation for allowing userspace to read/write the guest SSP
> register, which is needed for the upcoming CET virtualization support.
>
> Currently, two types of registers are supported: KVM_X86_REG_TYPE_MSR and
> KVM_X86_REG_TYPE_KVM. All MSRs are in the former type; the latter type is
> added for registers that lack existing KVM uAPIs to access them. The "KVM"
> in the name is intended to be vague to give KVM flexibility to include
> other potential registers.  More precise names like "SYNTHETIC" and
> "SYNTHETIC_MSR" were considered, but were deemed too confusing (e.g. can
> be conflated with synthetic guest-visible MSRs) and may put KVM into a
> corner (e.g. if KVM wants to change how a KVM-defined register is modeled
> internally).
>
> Enumerate only KVM-defined registers in KVM_GET_REG_LIST to avoid
> duplicating KVM_GET_MSR_INDEX_LIST, and so that KVM can return _only_
> registers that are fully supported (KVM_GET_REG_LIST is vCPU-scoped, i.e.
> can be precise, whereas KVM_GET_MSR_INDEX_LIST is system-scoped).
>
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> Link: https://lore.kernel.org/all/20240219074733.122080-18-weijiang.yang@xxxxxxxxx [1]
> 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: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>

One nit below.

> ---
>   Documentation/virt/kvm/api.rst  |   6 +-
>   arch/x86/include/uapi/asm/kvm.h |  26 +++++++++
>   arch/x86/kvm/x86.c              | 100 ++++++++++++++++++++++++++++++++
>   3 files changed, 131 insertions(+), 1 deletion(-)
[...]
> +
> +#define KVM_X86_REG_ENCODE(type, index)				\
Nit:
Is it better to use KVM_X86_REG_ID so that when searching with the string
non-case sensitively, the encoding and its structure can be related to each
other?


> +	(KVM_REG_X86 | KVM_X86_REG_TYPE_SIZE(type, index) | index)
> +
[...]
>   
> +struct kvm_x86_reg_id {
> +	__u32 index;
> +	__u8  type;
> +	__u8  rsvd1;
> +	__u8  rsvd2:4;
> +	__u8  size:4;
> +	__u8  x86;
> +};
> +