linux-kernel - Re: [PATCH Part2 RFC v2 36/37] KVM: SVM: Provide support for SNP_GUEST

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Date:   Mon, 10 May 2021 15:14:07 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Peter Gonda <pgonda@...gle.com>
Cc:     brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm list <kvm@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, jroedel@...e.de,
        "Lendacky, Thomas" <thomas.lendacky@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        David Rientjes <rientjes@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>, peterz@...radead.org,
        "H. Peter Anvin" <hpa@...or.com>, tony.luck@...el.com
Subject: Re: [PATCH Part2 RFC v2 36/37] KVM: SVM: Provide support for
 SNP_GUEST_REQUEST NAE event


On 5/10/21 1:57 PM, Peter Gonda wrote:
>> +static void snp_handle_guest_request(struct vcpu_svm *svm, struct ghcb *ghcb,
>> +                                   gpa_t req_gpa, gpa_t resp_gpa)
>> +{
>> +       struct sev_data_snp_guest_request data = {};
>> +       struct kvm_vcpu *vcpu = &svm->vcpu;
>> +       struct kvm *kvm = vcpu->kvm;
>> +       kvm_pfn_t req_pfn, resp_pfn;
>> +       struct kvm_sev_info *sev;
>> +       int rc, err = 0;
>> +
>> +       if (!sev_snp_guest(vcpu->kvm)) {
>> +               rc = -ENODEV;
>> +               goto e_fail;
>> +       }
>> +
>> +       sev = &to_kvm_svm(kvm)->sev_info;
>> +
>> +       if (!__ratelimit(&sev->snp_guest_msg_rs)) {
>> +               pr_info_ratelimited("svm: too many guest message requests\n");
>> +               rc = -EAGAIN;
>> +               goto e_fail;
>> +       }
>> +
>> +       if (!IS_ALIGNED(req_gpa, PAGE_SIZE) || !IS_ALIGNED(resp_gpa, PAGE_SIZE)) {
>> +               pr_err("svm: guest request (%#llx) or response (%#llx) is not page aligned\n",
>> +                       req_gpa, resp_gpa);
>> +               goto e_term;
>> +       }
>> +
>> +       req_pfn = gfn_to_pfn(kvm, gpa_to_gfn(req_gpa));
>> +       if (is_error_noslot_pfn(req_pfn)) {
>> +               pr_err("svm: guest request invalid gpa=%#llx\n", req_gpa);
>> +               goto e_term;
>> +       }
>> +
>> +       resp_pfn = gfn_to_pfn(kvm, gpa_to_gfn(resp_gpa));
>> +       if (is_error_noslot_pfn(resp_pfn)) {
>> +               pr_err("svm: guest response invalid gpa=%#llx\n", resp_gpa);
>> +               goto e_term;
>> +       }
>> +
>> +       data.gctx_paddr = __psp_pa(sev->snp_context);
>> +       data.req_paddr = __sme_set(req_pfn << PAGE_SHIFT);
>> +       data.res_paddr = __psp_pa(sev->snp_resp_page);
>> +
>> +       mutex_lock(&kvm->lock);
>> +
>> +       rc = sev_issue_cmd(kvm, SEV_CMD_SNP_GUEST_REQUEST, &data, &err);
>> +       if (rc) {
>> +               mutex_unlock(&kvm->lock);
>> +
>> +               /* If we have a firmware error code then use it. */
>> +               if (err)
>> +                       rc = err;
>> +
>> +               goto e_fail;
>> +       }
>> +
>> +       /* Copy the response after the firmware returns success. */
>> +       rc = kvm_write_guest(kvm, resp_gpa, sev->snp_resp_page, PAGE_SIZE);
>> +
>> +       mutex_unlock(&kvm->lock);
>> +
>> +e_fail:
>> +       ghcb_set_sw_exit_info_2(ghcb, rc);
>> +       return;
>> +
>> +e_term:
>> +       ghcb_set_sw_exit_info_1(ghcb, 1);
>> +       ghcb_set_sw_exit_info_2(ghcb,
>> +                               X86_TRAP_GP |
>> +                               SVM_EVTINJ_TYPE_EXEPT |
>> +                               SVM_EVTINJ_VALID);
>> +}
> I am probably missing something in the spec but I don't see any
> references to #GP in the '4.1.7 SNP Guest Request' section. Why is
> this different from e_fail?

The spec does not say to inject the #GP, I chose this because guest is
not adhering to the spec and there was a not a good error code in the
GHCB spec to communicate this condition. Per the spec, both the request
and response page must be a valid GPA. If we detect that guest is not
following the spec then its a guest BUG. IIRC, other places in the KVM
does something similar when guest is trying invalid operation.

-Brijesh