linux-kernel - Re: [PATCH Part2 RFC v4 26/40] KVM: SVM: Add KVM_SEV_SNP_LAUNCH

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Message-ID: <YPXPKLW8DvqK7yak@google.com>
Date:   Mon, 19 Jul 2021 19:14:48 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>, tony.luck@...el.com,
        npmccallum@...hat.com, brijesh.ksingh@...il.com
Subject: Re: [PATCH Part2 RFC v4 26/40] KVM: SVM: Add
 KVM_SEV_SNP_LAUNCH_FINISH command

On Mon, Jul 19, 2021, Brijesh Singh wrote:
> 
> 
> On 7/19/21 11:54 AM, Sean Christopherson wrote:
> > > As I said in previous comments that by default all the memory is in the
> > > hypervisor state. if the rmpupdate() failed that means nothing is changed in
> > > the RMP and there is no need to reclaim. The reclaim is required only if the
> > > pages are assigned in the RMP table.
> > 
> > I wasn't referring to RMPUPDATE failing here (or anywhere).  This is the vCPU free
> > path, which I think means the svm->vmsa page was successfully updated in the RMP
> > during LAUNCH_UPDATE.  snp_launch_update_vmsa() goes through snp_page_reclaim()
> > on LAUNCH_UPDATE failure, whereas this happy path does not.  Is there some other
> > transition during teardown that obviastes the need for reclaim?  If so, a comment
> > to explain that would be very helpful.
> > 
> 
> In this patch, the sev_free_vcpu() hunk takes care of reclaiming the vmsa
> pages before releasing it. I think it will make it more obvious after I add
> a helper so that we don't depend on user reading the comment block to see
> what its doing.

Where?  I feel like I'm missing something.  The only change to sev_free_vcpu() I
see is that addition of the rmpupdate(), I don't see any reclaim path.

@@ -2346,8 +2454,25 @@ void sev_free_vcpu(struct kvm_vcpu *vcpu)

        if (vcpu->arch.guest_state_protected)
                sev_flush_guest_memory(svm, svm->vmsa, PAGE_SIZE);
+
+       /*
+        * If its an SNP guest, then VMSA was added in the RMP entry as a guest owned page.
+        * Transition the page to hyperivosr state before releasing it back to the system.
+        */
+       if (sev_snp_guest(vcpu->kvm)) {
+               struct rmpupdate e = {};
+               int rc;
+
+               rc = rmpupdate(virt_to_page(svm->vmsa), &e);
+               if (rc) {
+                       pr_err("Failed to release SNP guest VMSA page (rc %d), leaking it\n", rc);
+                       goto skip_vmsa_free;
+               }
+       }
+
        __free_page(virt_to_page(svm->vmsa));

+skip_vmsa_free:
        if (svm->ghcb_sa_free)
                kfree(svm->ghcb_sa);
 }