linux-kernel - Re: [PATCH v4.1] KVM, SEV: Add KVM_EXIT

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Date:   Fri, 8 Apr 2022 02:55:50 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Peter Gonda <pgonda@...gle.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4.1] KVM, SEV: Add KVM_EXIT_SHUTDOWN metadata for SEV-ES

On Thu, Apr 07, 2022, Peter Gonda wrote:
> If an SEV-ES guest requests termination, exit to userspace with
> KVM_EXIT_SYSTEM_EVENT and a dedicated SEV_TERM type instead of -EINVAL
> so that userspace can take appropriate action.
> 
> See AMD's GHCB spec section '4.1.13 Termination Request' for more details.

Maybe it'll be obvious by the lack of compilation errors, but the changelog should
call out the flags => ndata+data shenanigans, otherwise this looks like ABI breakage.

> Suggested-by: Sean Christopherson <seanjc@...gle.com>
> Suggested-by: Paolo Bonzini <pbonzini@...hat.com>
> Cc: kvm@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Peter Gonda <pgonda@...gle.com>
> 
> ---
> V4
>  * Updated to Sean and Paolo's suggestion of reworking the
>    kvm_run.system_event struct to ndata and data fields to fix the
>    padding.
>  * 4.1 Updated commit description
> 
> V3
>  * Add Documentation/ update.
>  * Updated other KVM_EXIT_SHUTDOWN exits to clear ndata and set reason
>    to KVM_SHUTDOWN_REQ.
> 
> V2
>  * Add KVM_CAP_EXIT_SHUTDOWN_REASON check for KVM_CHECK_EXTENSION.
> 
> Tested by making an SEV-ES guest call sev_es_terminate() with hardcoded
> reason code set and reason code and then observing the codes from the
> userspace VMM in the kvm_run.shutdown.data fields.
> 
> ---
>  arch/x86/kvm/svm/sev.c   | 9 +++++++--
>  include/uapi/linux/kvm.h | 5 ++++-
>  2 files changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 75fa6dd268f0..1a080f3f09d8 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2735,8 +2735,13 @@ static int sev_handle_vmgexit_msr_protocol(struct vcpu_svm *svm)
>  		pr_info("SEV-ES guest requested termination: %#llx:%#llx\n",
>  			reason_set, reason_code);
>  
> -		ret = -EINVAL;
> -		break;
> +		vcpu->run->exit_reason = KVM_EXIT_SYSTEM_EVENT;
> +		vcpu->run->system_event.type = KVM_SYSTEM_EVENT_SEV_TERM |
> +					       KVM_SYSTEM_EVENT_NDATA_VALID;
> +		vcpu->run->system_event.ndata = 1;
> +		vcpu->run->system_event.data[1] = control->ghcb_gpa;
> +
> +		return 0;

Kinda silly, but

		ret = 0;
		break;

would be better so that this flows through the tracepoint.  I wouldn't care much
if it didn't result in an unpaired "entry" tracepoint (and I still don't care that
much...).

>  	}
>  	default:
>  		/* Error, keep GHCB MSR value as-is */
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 8616af85dc5d..dd1d8167e71f 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -444,8 +444,11 @@ struct kvm_run {
>  #define KVM_SYSTEM_EVENT_SHUTDOWN       1
>  #define KVM_SYSTEM_EVENT_RESET          2
>  #define KVM_SYSTEM_EVENT_CRASH          3
> +#define KVM_SYSTEM_EVENT_SEV_TERM       4
> +#define KVM_SYSTEM_EVENT_NDATA_VALID    (1u << 31)
>  			__u32 type;
> -			__u64 flags;
> +			__u32 ndata;
> +			__u64 data[16];
>  		} system_event;
>  		/* KVM_EXIT_S390_STSI */
>  		struct {
> -- 
> 2.35.1.1178.g4f1659d476-goog
>