Re: [PATCH RFC v7 47/64] KVM: SVM: Add support to handle MSR based Page State Change VMGEXIT

[Tom Dohrmann <erbse.13@gmx.de>]

On Wed, Dec 14, 2022 at 01:40:39PM -0600, Michael Roth wrote:
> From: Brijesh Singh <brijesh.singh@amd.com>
>
> SEV-SNP VMs can ask the hypervisor to change the page state in the RMP
> table to be private or shared using the Page State Change MSR protocol
> as defined in the GHCB specification.
>
> Forward these requests to userspace via KVM_EXIT_VMGEXIT so the VMM can
> issue the KVM ioctls to update the page state accordingly.
>
> Co-developed-by: Michael Roth <michael.roth@amd.com>
> Signed-off-by: Michael Roth <michael.roth@amd.com>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
> ---
>  arch/x86/include/asm/sev-common.h |  9 ++++++++
>  arch/x86/kvm/svm/sev.c            | 25 +++++++++++++++++++++++
>  arch/x86/kvm/trace.h              | 34 +++++++++++++++++++++++++++++++
>  arch/x86/kvm/x86.c                |  1 +
>  4 files changed, 69 insertions(+)
>
> diff --git a/arch/x86/include/asm/sev-common.h b/arch/x86/include/asm/sev-common.h
> index 0a9055cdfae2..ee38f7408470 100644
> --- a/arch/x86/include/asm/sev-common.h
> +++ b/arch/x86/include/asm/sev-common.h
> @@ -93,6 +93,10 @@ enum psc_op {
>  };
>
>  #define GHCB_MSR_PSC_REQ		0x014
> +#define GHCB_MSR_PSC_GFN_POS		12
> +#define GHCB_MSR_PSC_GFN_MASK		GENMASK_ULL(39, 0)
> +#define GHCB_MSR_PSC_OP_POS		52
> +#define GHCB_MSR_PSC_OP_MASK		0xf
>  #define GHCB_MSR_PSC_REQ_GFN(gfn, op)			\
>  	/* GHCBData[55:52] */				\
>  	(((u64)((op) & 0xf) << 52) |			\
> @@ -102,6 +106,11 @@ enum psc_op {
>  	GHCB_MSR_PSC_REQ)
>
>  #define GHCB_MSR_PSC_RESP		0x015
> +#define GHCB_MSR_PSC_ERROR_POS		32
> +#define GHCB_MSR_PSC_ERROR_MASK		GENMASK_ULL(31, 0)
> +#define GHCB_MSR_PSC_ERROR		GENMASK_ULL(31, 0)
> +#define GHCB_MSR_PSC_RSVD_POS		12
> +#define GHCB_MSR_PSC_RSVD_MASK		GENMASK_ULL(19, 0)
>  #define GHCB_MSR_PSC_RESP_VAL(val)			\
>  	/* GHCBData[63:32] */				\
>  	(((u64)(val) & GENMASK_ULL(63, 32)) >> 32)
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index d7b467b620aa..d7988629073b 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -29,6 +29,7 @@
>  #include "svm_ops.h"
>  #include "cpuid.h"
>  #include "trace.h"
> +#include "mmu.h"
>
>  #ifndef CONFIG_KVM_AMD_SEV
>  /*
> @@ -3350,6 +3351,23 @@ static void set_ghcb_msr(struct vcpu_svm *svm, u64 value)
>  	svm->vmcb->control.ghcb_gpa = value;
>  }
>
> +/*
> + * TODO: need to get the value set by userspace in vcpu->run->vmgexit.ghcb_msr
> + * and process that here accordingly.
> + */
> +static int snp_complete_psc_msr_protocol(struct kvm_vcpu *vcpu)
> +{
> +	struct vcpu_svm *svm = to_svm(vcpu);
> +
> +	set_ghcb_msr_bits(svm, 0,
> +			  GHCB_MSR_PSC_ERROR_MASK, GHCB_MSR_PSC_ERROR_POS);
> +
> +	set_ghcb_msr_bits(svm, 0, GHCB_MSR_PSC_RSVD_MASK, GHCB_MSR_PSC_RSVD_POS);
> +	set_ghcb_msr_bits(svm, GHCB_MSR_PSC_RESP, GHCB_MSR_INFO_MASK, GHCB_MSR_INFO_POS);
> +
> +	return 1; /* resume */
> +}
> +
>  static int sev_handle_vmgexit_msr_protocol(struct vcpu_svm *svm)
>  {
>  	struct vmcb_control_area *control = &svm->vmcb->control;
> @@ -3450,6 +3468,13 @@ static int sev_handle_vmgexit_msr_protocol(struct vcpu_svm *svm)
>  				  GHCB_MSR_INFO_POS);
>  		break;
>  	}
> +	case GHCB_MSR_PSC_REQ:
> +		vcpu->run->exit_reason = KVM_EXIT_VMGEXIT;
> +		vcpu->run->vmgexit.ghcb_msr = control->ghcb_gpa;
> +		vcpu->arch.complete_userspace_io = snp_complete_psc_msr_protocol;
> +
> +		ret = -1;
> +		break;

What's the reasoning behind returning an error (-1) here? This error bubbles all
the way up to the `KVM_RUN` ioctl. Would it be more appropriate to return 0?
Returning 0 would cause a VM exit without indicating an error to userspace.

>  	case GHCB_MSR_TERM_REQ: {
>  		u64 reason_set, reason_code;
>
> diff --git a/arch/x86/kvm/trace.h b/arch/x86/kvm/trace.h
> index 83843379813e..65861d2d086c 100644
> --- a/arch/x86/kvm/trace.h
> +++ b/arch/x86/kvm/trace.h
> @@ -7,6 +7,7 @@
>  #include <asm/svm.h>
>  #include <asm/clocksource.h>
>  #include <asm/pvclock-abi.h>
> +#include <asm/sev-common.h>
>
>  #undef TRACE_SYSTEM
>  #define TRACE_SYSTEM kvm
> @@ -1831,6 +1832,39 @@ TRACE_EVENT(kvm_vmgexit_msr_protocol_exit,
>  		  __entry->vcpu_id, __entry->ghcb_gpa, __entry->result)
>  );
>
> +/*
> + * Tracepoint for the SEV-SNP page state change processing
> + */
> +#define psc_operation					\
> +	{SNP_PAGE_STATE_PRIVATE, "private"},		\
> +	{SNP_PAGE_STATE_SHARED,  "shared"}		\
> +
> +TRACE_EVENT(kvm_snp_psc,
> +	TP_PROTO(unsigned int vcpu_id, u64 pfn, u64 gpa, u8 op, int level),
> +	TP_ARGS(vcpu_id, pfn, gpa, op, level),
> +
> +	TP_STRUCT__entry(
> +		__field(int, vcpu_id)
> +		__field(u64, pfn)
> +		__field(u64, gpa)
> +		__field(u8, op)
> +		__field(int, level)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->vcpu_id = vcpu_id;
> +		__entry->pfn = pfn;
> +		__entry->gpa = gpa;
> +		__entry->op = op;
> +		__entry->level = level;
> +	),
> +
> +	TP_printk("vcpu %u, pfn %llx, gpa %llx, op %s, level %d",
> +		  __entry->vcpu_id, __entry->pfn, __entry->gpa,
> +		  __print_symbolic(__entry->op, psc_operation),
> +		  __entry->level)
> +);
> +
>  #endif /* _TRACE_KVM_H */
>
>  #undef TRACE_INCLUDE_PATH
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 732f9cbbadb5..08dd1ef7e136 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -13481,6 +13481,7 @@ EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_vmgexit_enter);
>  EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_vmgexit_exit);
>  EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_vmgexit_msr_protocol_enter);
>  EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_vmgexit_msr_protocol_exit);
> +EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_snp_psc);
>
>  static int __init kvm_x86_init(void)
>  {
> --
> 2.25.1
>

Regards, Tom