linux-kernel - Re: [RFC] KVM: x86: Support KVM VMs sharing SEV context

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Date:   Thu, 25 Feb 2021 10:49:00 -0800
From:   Steve Rutherford <srutherford@...gle.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     Nathan Tempelman <natet@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, X86 ML <x86@...nel.org>,
        KVM list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Ashish Kalra <Ashish.Kalra@....com>
Subject: Re: [RFC] KVM: x86: Support KVM VMs sharing SEV context

On Thu, Feb 25, 2021 at 6:57 AM Tom Lendacky <thomas.lendacky@....com> wrote:
> >> +int svm_vm_copy_asid_to(struct kvm *kvm, unsigned int mirror_kvm_fd)
> >> +{
> >> +       struct file *mirror_kvm_file;
> >> +       struct kvm *mirror_kvm;
> >> +       struct kvm_sev_info *mirror_kvm_sev;
> >> +       unsigned int asid;
> >> +       int ret;
> >> +
> >> +       if (!sev_guest(kvm))
> >> +               return -ENOTTY;
> >
> > You definitely don't want this: this is the function that turns the vm
> > into an SEV guest (marks SEV as active).
>
> The sev_guest() function does not set sev->active, it only checks it. The
> sev_guest_init() function is where sev->active is set.
Sorry, bad use of the english on my part: the "this" was referring to
svm_vm_copy_asid_to. Right now, you could only pass this sev_guest
check if you had already called sev_guest_init, which seems incorrect.
>
> >
> > (Not an issue with this patch, but a broader issue) I believe
> > sev_guest lacks the necessary acquire/release barriers on sev->active,
>
> The svm_mem_enc_op() takes the kvm lock and that is the only way into the
> sev_guest_init() function where sev->active is set.
There are a few places that check sev->active which don't have the kvm
lock, which is not problematic if we add in a few compiler barriers
(ala irqchip_split et al).
>
> Thanks,
> Tom

Thanks,
Steve