linux-kernel - Re: [PATCH Part2 RFC v4 32/40] KVM: SVM: Add support to handle GHCB GPA register VMGEXIT

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Message-ID: <YPXa3gWrBW9fdGXK@google.com>
Date:   Mon, 19 Jul 2021 20:04:46 +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 32/40] KVM: SVM: Add support to handle GHCB
 GPA register VMGEXIT

On Fri, Jul 16, 2021, Brijesh Singh wrote:
> 
> On 7/16/21 3:45 PM, Sean Christopherson wrote:
> > On Wed, Jul 07, 2021, Brijesh Singh wrote:
> >> +	/* SEV-SNP guest requires that the GHCB GPA must be registered */
> >> +	if (sev_snp_guest(svm->vcpu.kvm) && !ghcb_gpa_is_registered(svm, ghcb_gpa)) {
> >> +		vcpu_unimpl(&svm->vcpu, "vmgexit: GHCB GPA [%#llx] is not registered.\n", ghcb_gpa);
> > I saw this a few other place.  vcpu_unimpl() is not the right API.  KVM supports
> > the guest request, the problem is that the GHCB spec _requires_ KVM to terminate
> > the guest in this case.
> 
> What is the preferred method to log it so that someone debugging know
> what went wrong.

Using the kernel log is probably a bad choice in general for this error.  Because
this and the other GHCB GPA sanity checks can be triggered from the guest, any
kernel logging needs to be ratelimited.  Ratelimiting is problematic because it
means some errors may not be logged; that's quite unlikely in this case, but it's
less than ideal.

The other issue is that KVM can't dump the RIP because guest state is encrypted,
e.g. KVM can provide the task PID, but that's it.

The best solution I can think of at the moment would be some form of
KVM_EXIT_INTERNAL_ERROR, i.e. kick out to userspace with a meaningful error code
and the bad GPA so that userspace can take action.

I believe Jim also has some thoughts on how to improve "logging" of guest errors,
but he's on vacation for a few weeks.