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

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Message-ID: <YPWuVY+rKU2/DVUS@google.com>
Date:   Mon, 19 Jul 2021 16:54:45 +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 Fri, Jul 16, 2021, Brijesh Singh wrote:
> 
> On 7/16/21 3:18 PM, Sean Christopherson wrote:
> > On Wed, Jul 07, 2021, Brijesh Singh wrote:
> >> +	data->gctx_paddr = __psp_pa(sev->snp_context);
> >> +	ret = sev_issue_cmd(kvm, SEV_CMD_SNP_LAUNCH_FINISH, data, &argp->error);
> > Shouldn't KVM unwind everything it did if LAUNCH_FINISH fails?  And if that's
> > not possible, take steps to make the VM unusable?
> 
> Well, I am not sure if VM need to unwind. If the command fail but VMM decide
> to ignore the error then VMRUN will probably fail and user will get the KVM
> shutdown event. The LAUNCH_FINISH command finalizes the VM launch process,
> the firmware will probably not load the memory encryption keys until it moves
> to the running state.

Within reason, KVM needs to provide consistent, deterministic behavior.  Yes, more
than likely failure at this point will be fatal to the VM, but that doesn't justify
leaving the VM in a random/bogus state.  In addition to being a poor ABI, it also
makes it more difficult to reason about what is/isn't possible in KVM.

> >> +	 */
> >> +	if (sev_snp_guest(vcpu->kvm)) {
> >> +		struct rmpupdate e = {};
> >> +		int rc;
> >> +
> >> +		rc = rmpupdate(virt_to_page(svm->vmsa), &e);
> > So why does this not need to go through snp_page_reclaim()?
> 
> 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.