From: Mike Larkin <mlarkin@nested.page>
Subject: Re: SEV-ES: vmm(4): GHCB MSR protocol for CPUID
To: tech@openbsd.org
Date: Tue, 20 May 2025 17:58:45 -0700

On Tue, May 20, 2025 at 02:05:28PM +0200, Hans-Jörg Höxer wrote:
> Hi,
>
> my last two diffs for vmm(4) and SEV-ES:
>
>     When a SEV-ES guest is in locore, it will not be able yet to use a
>     GHCB to communicate with vmm(4).  Therefore, AMD specifies a "GHCB
>     MSR protocol" that uses the lowere 12 bits of the GHCB MSR to request
>     services from vmm(4).  Guest writes to the GHCB MSR will show up in
>     the v_ghcb_gpa member of the VMCB and are thus accesible by vmm(4).
>     The response of vmm(4) can be provided by writing it to the VMCB.
>
>     In locore a SEV-ES guest will need to request CPUID from vmm(4)
>     using the GHCB MSR protocol.  This diff provides this service.
>
> Take care,
> HJ.
>

Diff looks ok; one question though. Is the locore side of this diff still
coming or did that already go in? Eg, is there a corresponding change for
the guest to use the GHCB MSR protocol?

ok mlarkin in any case.

> ---------------------------------------------------------------------------
> commit 8278f4051b90ee94e19db25d019100a736ee7317
> Author: Hans-Joerg Hoexer <hshoexer@genua.de>
> Date:   Mon Nov 18 13:59:19 2024 +0100
>
>     vmm(4): GHCB MSR protocol for CPUID
>
>     When a SEV-ES guest is in locore, it will not be able yet to use a
>     GHCB to communicate with vmm(4).  Therefore, AMD specifies a "GHCB
>     MSR protocol" that uses the lowere 12 bits of the GHCB MSR to request
>     services from vmm(4).  Guest writes to the GHCB MSR will show up in
>     the v_ghcb_gpa member of the VMCB and are thus accesible by vmm(4).
>     The response of vmm(4) can be provided by writing it to the VMCB.
>
>     In locore a SEV-ES guest will need to request CPUID from vmm(4)
>     using the GHCB MSR protocol.  This diff provides this service.
>
> diff --git a/sys/arch/amd64/amd64/vmm_machdep.c b/sys/arch/amd64/amd64/vmm_machdep.c
> index 9049bc6340d..2e90b7340f2 100644
> --- a/sys/arch/amd64/amd64/vmm_machdep.c
> +++ b/sys/arch/amd64/amd64/vmm_machdep.c
> @@ -4514,13 +4514,67 @@ svm_handle_gexit(struct vcpu *vcpu)
>  	struct vm		*vm = vcpu->vc_parent;
>  	struct ghcb_sa		*ghcb;
>  	paddr_t			 ghcb_gpa, ghcb_hpa;
> +	uint32_t		 req, resp;
> +	uint64_t		 result;
>  	int			 syncout, error = 0;
>
> -	if (vcpu->vc_svm_ghcb_va == 0) {
> +	if (vcpu->vc_svm_ghcb_va == 0 && (vmcb->v_ghcb_gpa & ~PG_FRAME) == 0 &&
> +	    (vmcb->v_ghcb_gpa & PG_FRAME) != 0) {
> +		/*
> +		 * Guest provides a valid guest physcial address
> +		 * for GHCB and it is not set yet -> assign it.
> +		 *
> +		 * We only accept a GHCB once; we decline re-definition.
> +		 */
>  		ghcb_gpa = vmcb->v_ghcb_gpa & PG_FRAME;
>  		if (!pmap_extract(vm->vm_map->pmap, ghcb_gpa, &ghcb_hpa))
>  			return (EINVAL);
>  		vcpu->vc_svm_ghcb_va = (vaddr_t)PMAP_DIRECT_MAP(ghcb_hpa);
> +	} else if ((vmcb->v_ghcb_gpa & ~PG_FRAME) != 0) {
> +		/*
> +                 * Low bits in use, thus must be a MSR protocol
> +		 * request.
> +		 */
> +		req = (vmcb->v_ghcb_gpa & 0xffffffff);
> +
> +		/* we only support cpuid */
> +		if ((req & ~PG_FRAME) != MSR_PROTO_CPUID_REQ)
> +			return (EINVAL);
> +
> +		/* Emulate CPUID */
> +		vmcb->v_exitcode = SVM_VMEXIT_CPUID;
> +		vmcb->v_rax = vmcb->v_ghcb_gpa >> 32;
> +		vcpu->vc_gueststate.vg_rax = 0;
> +		vcpu->vc_gueststate.vg_rbx = 0;
> +		vcpu->vc_gueststate.vg_rcx = 0;
> +		vcpu->vc_gueststate.vg_rdx = 0;
> +		error = vmm_handle_cpuid(vcpu);
> +		if (error)
> +			goto out;
> +
> +		switch (req >> 30) {
> +		case 0:	/* eax: emulate cpuid and return eax */
> +			result = vmcb->v_rax;
> +			break;
> +		case 1:	/* return ebx */
> +			result = vcpu->vc_gueststate.vg_rbx;
> +			break;
> +		case 2: /* return ecx */
> +			result = vcpu->vc_gueststate.vg_rcx;
> +			break;
> +		case 3:	/* return edx */
> +			result = vcpu->vc_gueststate.vg_rdx;
> +			break;
> +		default:
> +			DPRINTF("%s: unknown request 0x%x\n", __func__, req);
> +			return (EINVAL);
> +		}
> +
> +		/* build response */
> +		resp = MSR_PROTO_CPUID_RESP | (req & 0xc0000000);
> +		vmcb->v_ghcb_gpa = (result << 32) | resp;
> +
> +		return (0);
>  	}
>
>  	/* Verify GHCB and synchronize guest state information. */
> diff --git a/sys/arch/amd64/include/ghcb.h b/sys/arch/amd64/include/ghcb.h
> index 802bf7f015e..954e1fa3e3b 100644
> --- a/sys/arch/amd64/include/ghcb.h
> +++ b/sys/arch/amd64/include/ghcb.h
> @@ -101,6 +101,12 @@ struct ghcb_sync {
>  };
>
>
> +/* Definitions used with the MSR protocol */
> +#define MSR_PROTO_CPUID_REQ	0x4
> +#define MSR_PROTO_CPUID_RESP	0x5
> +#define MSR_PROTO_TERMINATE	0x100
> +
> +
>  void	ghcb_clear(struct ghcb_sa *);
>  int	ghcb_valbm_set(uint8_t *, int);
>  int	ghcb_valbm_isset(uint8_t *, int);