linux-kernel - Re: [PATCH RFC v7 07/64] KVM: SEV: Handle KVM_HC_MAP_GPA

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Message-ID: <20230127163558.GA22533@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
Date:   Fri, 27 Jan 2023 08:35:58 -0800
From:   Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
To:     Michael Roth <michael.roth@....com>
Cc:     kvm@...r.kernel.org, linux-coco@...ts.linux.dev,
        linux-mm@...ck.org, linux-crypto@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        jroedel@...e.de, thomas.lendacky@....com, hpa@...or.com,
        ardb@...nel.org, pbonzini@...hat.com, seanjc@...gle.com,
        vkuznets@...hat.com, wanpengli@...cent.com, jmattson@...gle.com,
        luto@...nel.org, dave.hansen@...ux.intel.com, slp@...hat.com,
        pgonda@...gle.com, peterz@...radead.org,
        srinivas.pandruvada@...ux.intel.com, rientjes@...gle.com,
        dovmurik@...ux.ibm.com, tobin@....com, bp@...en8.de,
        vbabka@...e.cz, kirill@...temov.name, ak@...ux.intel.com,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, alpergun@...gle.com,
        dgilbert@...hat.com, jarkko@...nel.org, ashish.kalra@....com,
        harald@...fian.com, Nikunj A Dadhania <nikunj@....com>
Subject: Re: [PATCH RFC v7 07/64] KVM: SEV: Handle KVM_HC_MAP_GPA_RANGE
 hypercall

On Wed, Dec 14, 2022 at 01:39:59PM -0600, Michael Roth wrote:
> From: Nikunj A Dadhania <nikunj@....com>
> 
> KVM_HC_MAP_GPA_RANGE hypercall is used by the SEV guest to notify a
> change in the page encryption status to the hypervisor.
> 
> The hypercall exits to userspace with KVM_EXIT_HYPERCALL exit code,
> currently this is used for explicit memory conversion between
> shared/private for memfd based private memory.
> 
> Signed-off-by: Nikunj A Dadhania <nikunj@....com>
> Signed-off-by: Michael Roth <michael.roth@....com>
> ---
>  arch/x86/kvm/x86.c  | 8 ++++++++
>  virt/kvm/kvm_main.c | 1 +
>  2 files changed, 9 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index bb6adb216054..732f9cbbadb5 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9649,6 +9649,7 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)

Couldn't find a better commit to comment on:
when the guest has the ptp-kvm module, it will issue a KVM_HC_CLOCK_PAIRING
hypercall. This will pass sev_es_validate_vmgexit validation and end up in this
function where kvm_pv_clock_pairing() is called, and that calls
kvm_write_guest(). This results in a CPU soft-lockup, at least in my testing.

Are there any emulated hypercalls that make sense for snp guests? We should
block at least the ones that definitely don't work.

Jeremi

>  		break;
>  	case KVM_HC_MAP_GPA_RANGE: {
>  		u64 gpa = a0, npages = a1, attrs = a2;
> +		struct kvm_memory_slot *slot;
>  
>  		ret = -KVM_ENOSYS;
>  		if (!(vcpu->kvm->arch.hypercall_exit_enabled & (1 << KVM_HC_MAP_GPA_RANGE)))
> @@ -9660,6 +9661,13 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>  			break;
>  		}
>  
> +		slot = kvm_vcpu_gfn_to_memslot(vcpu, gpa_to_gfn(gpa));
> +		if (!vcpu->kvm->arch.upm_mode ||
> +		    !kvm_slot_can_be_private(slot)) {
> +			ret = 0;
> +			break;
> +		}
> +
>  		vcpu->run->exit_reason        = KVM_EXIT_HYPERCALL;
>  		vcpu->run->hypercall.nr       = KVM_HC_MAP_GPA_RANGE;
>  		vcpu->run->hypercall.args[0]  = gpa;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index d2daa049e94a..73bf0bdedb59 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2646,6 +2646,7 @@ struct kvm_memory_slot *kvm_vcpu_gfn_to_memslot(struct kvm_vcpu *vcpu, gfn_t gfn
>  
>  	return NULL;
>  }
> +EXPORT_SYMBOL_GPL(kvm_vcpu_gfn_to_memslot);
>  
>  bool kvm_is_visible_gfn(struct kvm *kvm, gfn_t gfn)
>  {
> -- 
> 2.25.1
>