linux-kernel - Re: [PATCH v10 48/50] KVM: SEV: Provide support for SNP_GUEST

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Message-ID: <2034624b-579f-482e-8a7a-0dfc91740d7e@amd.com>
Date:   Fri, 20 Oct 2023 10:55:18 +1100
From:   Alexey Kardashevskiy <aik@....com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Dionna Amalie Glaze <dionnaglaze@...gle.com>,
        Michael Roth <michael.roth@....com>, 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, vkuznets@...hat.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,
        jarkko@...nel.org, ashish.kalra@....com, nikunj.dadhania@....com,
        pankaj.gupta@....com, liam.merwick@...cle.com,
        zhi.a.wang@...el.com, Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v10 48/50] KVM: SEV: Provide support for SNP_GUEST_REQUEST
 NAE event


On 20/10/23 01:57, Sean Christopherson wrote:
> On Thu, Oct 19, 2023, Alexey Kardashevskiy wrote:
>>
>> On 19/10/23 00:48, Sean Christopherson wrote:
>>> static int snp_handle_ext_guest_request(struct vcpu_svm *svm)
>>> {
>>> 	struct kvm_vcpu *vcpu = &svm->vcpu;
>>> 	struct kvm *kvm = vcpu->kvm;
>>> 	struct kvm_sev_info *sev;
>>> 	unsigned long exitcode;
>>> 	u64 data_gpa;
>>>
>>> 	if (!sev_snp_guest(vcpu->kvm)) {
>>> 		ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, SEV_RET_INVALID_GUEST);
>>> 		return 1;
>>> 	}
>>>
>>> 	data_gpa = vcpu->arch.regs[VCPU_REGS_RAX];
>>> 	if (!IS_ALIGNED(data_gpa, PAGE_SIZE)) {
>>> 		ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, SEV_RET_INVALID_ADDRESS);
>>> 		return 1;
>>> 	}
>>>
>>> 	vcpu->run->hypercall.nr		 = KVM_HC_SNP_GET_CERTS;
>>> 	vcpu->run->hypercall.args[0]	 = data_gpa;
>>> 	vcpu->run->hypercall.args[1]	 = vcpu->arch.regs[VCPU_REGS_RBX];
>>> 	vcpu->run->hypercall.flags	 = KVM_EXIT_HYPERCALL_LONG_MODE;
>>
>> btw why is it _LONG_MODE and not just _64? :)
> 
> I'm pretty sure it got copied from Xen when KVM started adding supporting for
> emulating Xen's hypercalls.  I assume Xen PV actually has a need for identifying
> long mode as opposed to just 64-bit mode, but KVM, not so much.
> 
>>> 	vcpu->arch.complete_userspace_io = snp_complete_ext_guest_request;
>>> 	return 0;
>>> }
>>
>> This should work the KVM stored certs nicely but not for the global certs.
>> Although I am not all convinced that global certs is all that valuable but I
>> do not know the history of that, happened before I joined so I let others to
>> comment on that. Thanks,
> 
> Aren't the global certs provided by userspace too though?  If all certs are
> ultimately controlled by userspace, I don't see any reason to make the kernel a
> middle-man.

The max blob size is 32KB or so and for 200 VMs it is:
- 6.5MB, all in the userspace so swappable  vs
- 32KB but in the kernel so not swappable.
Sure, a box capable of running 200 VMs must have plenty of RAM but still :)
Plus, GHCB now has to go via the userspace before talking to the PSP 
which was not the case so far (though I cannot think of immediate 
implication right now).


-- 
Alexey