linux-kernel - Re: [PATCH v11 08/13] KVM: X86: Introduce KVM_HC_PAGE_ENC

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Date:   Tue, 6 Apr 2021 15:20:21 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ashish Kalra <ashish.kalra@....com>
Cc:     Steve Rutherford <srutherford@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Joerg Roedel <joro@...tes.org>,
        Borislav Petkov <bp@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        X86 ML <x86@...nel.org>, KVM list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Venu Busireddy <venu.busireddy@...cle.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Will Deacon <will@...nel.org>, maz@...nel.org,
        Quentin Perret <qperret@...gle.com>
Subject: Re: [PATCH v11 08/13] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS
 hypercall

On Tue, Apr 06, 2021, Sean Christopherson wrote:
> On Tue, Apr 06, 2021, Ashish Kalra wrote:
> > On Mon, Apr 05, 2021 at 01:42:42PM -0700, Steve Rutherford wrote:
> > > On Mon, Apr 5, 2021 at 7:28 AM Ashish Kalra <Ashish.Kalra@....com> wrote:
> > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > > index f7d12fca397b..ef5c77d59651 100644
> > > > --- a/arch/x86/kvm/x86.c
> > > > +++ b/arch/x86/kvm/x86.c
> > > > @@ -8273,6 +8273,18 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> > > >                 kvm_sched_yield(vcpu->kvm, a0);
> > > >                 ret = 0;
> > > >                 break;
> > > > +       case KVM_HC_PAGE_ENC_STATUS: {
> > > > +               int r;
> > > > +
> > > > +               ret = -KVM_ENOSYS;
> > > > +               if (kvm_x86_ops.page_enc_status_hc) {
> > > > +                       r = kvm_x86_ops.page_enc_status_hc(vcpu, a0, a1, a2);
> > > > +                       if (r >= 0)
> > > > +                               return r;
> > > > +                       ret = r;
> > > Style nit: Why not just set ret, and return ret if ret >=0?
> > > 
> > 
> > But ret is "unsigned long", if i set ret and return, then i will return to guest
> > even in case of error above ?
> 
> As proposed, svm_page_enc_status_hc() already hooks complete_userspace_io(), so
> this could be hoisted out of the switch statement.
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 16fb39503296..794dde3adfab 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8261,6 +8261,10 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>                 goto out;
>         }
> 
> +       /* comment goes here */
> +       if (nr == KVM_HC_PAGE_ENC_STATUS && kvm_x86_ops.page_enc_status_hc)
> +               return static_call(kvm_x86_page_enc_status_hc(vcpu, a0, a1, a2));

Gah, the SEV implementation can also return -EINVAL, and that should fail the
hypercall, not kill the guest.  Normally we try to avoid output params, but
in this case it might be less ugly to do:

		case KVM_HC_PAGE_ENC_STATUS: {
			if (!kvm_x86_ops.page_enc_status_hc)
				break;

			if (!static_call(kvm_x86_page_enc_status_hc(vcpu, a0, a1,
								    a2, &ret));
				return 0;
			break;

> +
>         ret = -KVM_ENOSYS;
> 
>         switch (nr) {
>