linux-kernel - Re: [PATCH 1/3 V7] KVM, SEV: Add support for SEV intra host migration

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Date:   Thu, 9 Sep 2021 18:15:56 -0700
From:   Marc Orr <marcorr@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Peter Gonda <pgonda@...gle.com>, kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3 V7] KVM, SEV: Add support for SEV intra host migration

> > +     dst->asid = src->asid;
> > +     dst->misc_cg = src->misc_cg;
> > +     dst->handle = src->handle;
> > +     dst->pages_locked = src->pages_locked;
> > +
> > +     src->asid = 0;
> > +     src->active = false;
> > +     src->handle = 0;
> > +     src->pages_locked = 0;
> > +     src->misc_cg = NULL;
> > +
> > +     INIT_LIST_HEAD(&dst->regions_list);
> > +     list_replace_init(&src->regions_list, &dst->regions_list);
> > +}
> > +
> > +int svm_vm_migrate_from(struct kvm *kvm, unsigned int source_fd)
> > +{
> > +     struct kvm_sev_info *dst_sev = &to_kvm_svm(kvm)->sev_info;
> > +     struct file *source_kvm_file;
> > +     struct kvm *source_kvm;
> > +     int ret;
> > +
> > +     ret = svm_sev_lock_for_migration(kvm);
> > +     if (ret)
> > +             return ret;
> > +
> > +     if (!sev_guest(kvm) || sev_es_guest(kvm)) {
> > +             ret = -EINVAL;
> > +             pr_warn_ratelimited("VM must be SEV enabled to migrate to.\n");
>
> Linux generally doesn't log user errors to dmesg.  They can be helpful during
> development, but aren't actionable and thus are of limited use in production.

Ha. I had suggested adding the logs when I reviewed these patches
(maybe before Peter posted them publicly). My rationale is that if I'm
looking at a crash in production, and all I have is a stack trace and
the error code, then I can narrow the failure down to this function,
but once the function starts returning the same error code in multiple
places now it's non-trivial for me to deduce exactly which condition
caused the crash. Having these logs makes it trivial. However, if this
is not the preferred Linux style then so be it.