linux-kernel - Re: [PATCH 08/12] KVM: X86: Introduce KVM_HC_PAGE_ENC

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Date:   Thu, 20 Feb 2020 21:21:20 +0000
From:   Ashish Kalra <ashish.kalra@....com>
To:     Steve Rutherford <srutherford@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        David Rientjes <rientjes@...gle.com>, x86@...nel.org,
        KVM list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, brijesh.singh@....com
Subject: Re: [PATCH 08/12] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS
 hypercall

On Thu, Feb 20, 2020 at 05:28:21AM +0000, Ashish Kalra wrote:
> On Wed, Feb 19, 2020 at 06:39:39PM -0800, Steve Rutherford wrote:
> > On Wed, Feb 12, 2020 at 5:17 PM Ashish Kalra <Ashish.Kalra@....com> wrote:
> > >
> > >  static void vmx_cleanup_l1d_flush(void)
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index fbabb2f06273..298627fa3d39 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -7547,6 +7547,12 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> > >                 kvm_sched_yield(vcpu->kvm, a0);
> > >                 ret = 0;
> > >                 break;
> > > +       case KVM_HC_PAGE_ENC_STATUS:
> > > +               ret = -KVM_ENOSYS;
> > > +               if (kvm_x86_ops->page_enc_status_hc)
> > > +                       ret = kvm_x86_ops->page_enc_status_hc(vcpu->kvm,
> > > +                                       a0, a1, a2);
> > > +               break;
> > >         default:
> > >                 ret = -KVM_ENOSYS;
> > >                 break;
> > Add a cap to kvm_vm_ioctl_enable_cap so that the vmm can configure
> > whether or not this hypercall is offered. Moving to an enable cap
> > would also allow the vmm to pass down the expected size of the c-bit
> > tracking buffer, so that you don't need to handle dynamic resizing in
> > response to guest hypercall, otherwise KVM will sporadically start
> > copying around large buffers when working with large VMs.
> > 
> 
> Yes, that is something we have been looking at adding.
> 
> But, how will VMM know the expected size of the c-bit tracking buffer ?
> 
> The guest kernel and firmware make the hypercall to mark page encryption
> status and depending on the GPA range being marked, the kernel's page
> encryption bitmap needs to be dynamically resized as response to the guest
> hypercall.
> 

Discussed this with Brijesh, though KVM can provide a hint about
the expected (max.) size of the c-bit tracking buffer, but there is
still an issue for hotplugged guest memory, hence dynamically sized 
encryption bitmap is probably the right approach.

Thanks,
Ashish