Message-ID: <20210206054617.GA19422@ashkalra_ubuntu_server>
Date:   Sat, 6 Feb 2021 05:46:17 +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>,
        X86 ML <x86@...nel.org>, KVM list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Venu Busireddy <venu.busireddy@...cle.com>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v10 12/16] KVM: x86: Introduce new
 KVM_FEATURE_SEV_LIVE_MIGRATION feature & Custom MSR.

Hello Steve,

Continued response to your queries, especially related to userspace
control of SEV live migration feature : 

On Fri, Feb 05, 2021 at 06:54:21PM -0800, Steve Rutherford wrote:
> On Thu, Feb 4, 2021 at 7:08 PM Ashish Kalra <ashish.kalra@....com> wrote:
> >
> > Hello Steve,
> >
> > On Thu, Feb 04, 2021 at 04:56:35PM -0800, Steve Rutherford wrote:
> > > On Wed, Feb 3, 2021 at 4:39 PM Ashish Kalra <Ashish.Kalra@....com> wrote:
> > > >
> > > > From: Ashish Kalra <ashish.kalra@....com>
> > > >
> > > > Add new KVM_FEATURE_SEV_LIVE_MIGRATION feature for guest to check
> > > > for host-side support for SEV live migration. Also add a new custom
> > > > MSR_KVM_SEV_LIVE_MIGRATION for guest to enable the SEV live migration
> > > > feature.
> > > >
> > > > Signed-off-by: Ashish Kalra <ashish.kalra@....com>
> > > > ---
> > > >  Documentation/virt/kvm/cpuid.rst     |  5 +++++
> > > >  Documentation/virt/kvm/msr.rst       | 12 ++++++++++++
> > > >  arch/x86/include/uapi/asm/kvm_para.h |  4 ++++
> > > >  arch/x86/kvm/svm/sev.c               | 13 +++++++++++++
> > > >  arch/x86/kvm/svm/svm.c               | 16 ++++++++++++++++
> > > >  arch/x86/kvm/svm/svm.h               |  2 ++
> > > >  6 files changed, 52 insertions(+)
> > > >
> > > > diff --git a/Documentation/virt/kvm/cpuid.rst b/Documentation/virt/kvm/cpuid.rst
> > > > index cf62162d4be2..0bdb6cdb12d3 100644
> > > > --- a/Documentation/virt/kvm/cpuid.rst
> > > > +++ b/Documentation/virt/kvm/cpuid.rst
> > > > @@ -96,6 +96,11 @@ KVM_FEATURE_MSI_EXT_DEST_ID        15          guest checks this feature bit
> > > >                                                 before using extended destination
> > > >                                                 ID bits in MSI address bits 11-5.
> > > >
> > > > +KVM_FEATURE_SEV_LIVE_MIGRATION     16          guest checks this feature bit before
> > > > +                                               using the page encryption state
> > > > +                                               hypercall to notify the page state
> > > > +                                               change
> > > > +
> > > >  KVM_FEATURE_CLOCKSOURCE_STABLE_BIT 24          host will warn if no guest-side
> > > >                                                 per-cpu warps are expected in
> > > >                                                 kvmclock
> > > > diff --git a/Documentation/virt/kvm/msr.rst b/Documentation/virt/kvm/msr.rst
> > > > index e37a14c323d2..020245d16087 100644
> > > > --- a/Documentation/virt/kvm/msr.rst
> > > > +++ b/Documentation/virt/kvm/msr.rst
> > > > @@ -376,3 +376,15 @@ data:
> > > >         write '1' to bit 0 of the MSR, this causes the host to re-scan its queue
> > > >         and check if there are more notifications pending. The MSR is available
> > > >         if KVM_FEATURE_ASYNC_PF_INT is present in CPUID.
> > > > +
> > > > +MSR_KVM_SEV_LIVE_MIGRATION:
> > > > +        0x4b564d08
> > > > +
> > > > +       Control SEV Live Migration features.
> > > > +
> > > > +data:
> > > > +        Bit 0 enables (1) or disables (0) host-side SEV Live Migration feature,
> > > > +        in other words, this is guest->host communication that it's properly
> > > > +        handling the shared pages list.
> > > > +
> > > > +        All other bits are reserved.
> > > > diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
> > > > index 950afebfba88..f6bfa138874f 100644
> > > > --- a/arch/x86/include/uapi/asm/kvm_para.h
> > > > +++ b/arch/x86/include/uapi/asm/kvm_para.h
> > > > @@ -33,6 +33,7 @@
> > > >  #define KVM_FEATURE_PV_SCHED_YIELD     13
> > > >  #define KVM_FEATURE_ASYNC_PF_INT       14
> > > >  #define KVM_FEATURE_MSI_EXT_DEST_ID    15
> > > > +#define KVM_FEATURE_SEV_LIVE_MIGRATION 16
> > > >
> > > >  #define KVM_HINTS_REALTIME      0
> > > >
> > > > @@ -54,6 +55,7 @@
> > > >  #define MSR_KVM_POLL_CONTROL   0x4b564d05
> > > >  #define MSR_KVM_ASYNC_PF_INT   0x4b564d06
> > > >  #define MSR_KVM_ASYNC_PF_ACK   0x4b564d07
> > > > +#define MSR_KVM_SEV_LIVE_MIGRATION     0x4b564d08
> > > >
> > > >  struct kvm_steal_time {
> > > >         __u64 steal;
> > > > @@ -136,4 +138,6 @@ struct kvm_vcpu_pv_apf_data {
> > > >  #define KVM_PV_EOI_ENABLED KVM_PV_EOI_MASK
> > > >  #define KVM_PV_EOI_DISABLED 0x0
> > > >
> > > > +#define KVM_SEV_LIVE_MIGRATION_ENABLED BIT_ULL(0)
> > > > +
> > > >  #endif /* _UAPI_ASM_X86_KVM_PARA_H */
> > > > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> > > > index b0d324aed515..93f42b3d3e33 100644
> > > > --- a/arch/x86/kvm/svm/sev.c
> > > > +++ b/arch/x86/kvm/svm/sev.c
> > > > @@ -1627,6 +1627,16 @@ int svm_page_enc_status_hc(struct kvm *kvm, unsigned long gpa,
> > > >         return ret;
> > > >  }
> > > >
> > > > +void sev_update_migration_flags(struct kvm *kvm, u64 data)
> > > > +{
> > > > +       struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> > > > +
> > > > +       if (!sev_guest(kvm))
> > > > +               return;
> > >
> > > This should assert that userspace wanted the guest to be able to make
> > > these calls (see more below).
> > >
> > > >
> > > > +
> > > > +       sev->live_migration_enabled = !!(data & KVM_SEV_LIVE_MIGRATION_ENABLED);
> > > > +}
> > > > +
> > > >  int svm_get_shared_pages_list(struct kvm *kvm,
> > > >                               struct kvm_shared_pages_list *list)
> > > >  {
> > > > @@ -1639,6 +1649,9 @@ int svm_get_shared_pages_list(struct kvm *kvm,
> > > >         if (!sev_guest(kvm))
> > > >                 return -ENOTTY;
> > > >
> > > > +       if (!sev->live_migration_enabled)
> > > > +               return -EINVAL;
> 
> This is currently under guest control, so I'm not certain this is
> helpful. If I called this with otherwise valid parameters, and got
> back -EINVAL, I would probably think the bug is on my end. But it
> could be on the guest's end! I would probably drop this, but you could
> have KVM return an empty list of regions when this happens.
> 
> Alternatively, as explained below, this could call guest_pv_has instead.
> 
> >
> > > > +
> > > >         if (!list->size)
> > > >                 return -EINVAL;
> > > >
> > > > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > > > index 58f89f83caab..43ea5061926f 100644
> > > > --- a/arch/x86/kvm/svm/svm.c
> > > > +++ b/arch/x86/kvm/svm/svm.c
> > > > @@ -2903,6 +2903,9 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
> > > >                 svm->msr_decfg = data;
> > > >                 break;
> > > >         }
> > > > +       case MSR_KVM_SEV_LIVE_MIGRATION:
> > > > +               sev_update_migration_flags(vcpu->kvm, data);
> > > > +               break;
> > > >         case MSR_IA32_APICBASE:
> > > >                 if (kvm_vcpu_apicv_active(vcpu))
> > > >                         avic_update_vapic_bar(to_svm(vcpu), data);
> > > > @@ -3976,6 +3979,19 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> > > >                         vcpu->arch.cr3_lm_rsvd_bits &= ~(1UL << (best->ebx & 0x3f));
> > > >         }
> > > >
> > > > +       /*
> > > > +        * If SEV guest then enable the Live migration feature.
> > > > +        */
> > > > +       if (sev_guest(vcpu->kvm)) {
> > > > +               struct kvm_cpuid_entry2 *best;
> > > > +
> > > > +               best = kvm_find_cpuid_entry(vcpu, KVM_CPUID_FEATURES, 0);
> > > > +               if (!best)
> > > > +                       return;
> > > > +
> > > > +               best->eax |= (1 << KVM_FEATURE_SEV_LIVE_MIGRATION);
> > > > +       }
> > > > +
> > >
> > > Looking at this, I believe the only way for this bit to get enabled is
> > > if userspace toggles it. There needs to be a way for userspace to
> > > identify if the kernel underneath them does, in fact, support SEV LM.
> > > I'm at risk for having misread these patches (it's a long series), but
> > > I don't see anything that communicates upwards.
> > >
> > > This could go upward with the other paravirt features flags in
> > > cpuid.c. It could also be an explicit KVM Capability (checked through
> > > check_extension).
> > >
> > > Userspace should then have a chance to decide whether or not this
> > > should be enabled. And when it's not enabled, the host should return a
> > > GP in response to the hypercall. This could be configured either
> > > through userspace stripping out the LM feature bit, or by calling a VM
> > > scoped enable cap (KVM_VM_IOCTL_ENABLE_CAP).
> > >
> > > I believe the typical path for a feature like this to be configured
> > > would be to use ENABLE_CAP.
> >
> > I believe we have discussed and reviewed this earlier too.
> >
> > To summarize this feature, the host indicates if it supports the Live
> > Migration feature and the feature and the hypercall are only enabled on
> > the host when the guest checks for this support and does a wrmsrl() to
> > enable the feature. Also the guest will not make the hypercall if the
> > host does not indicate support for it.
> 
> I've gone through and read this patch a bit more closely, and the
> surrounding code. Previously, I clearly misread this and the
> surrounding space.
> 
> What happens if the guest just writes to the MSR anyway? Even if it
> didn't receive a cue to do so? I believe the hypercall would still get
> invoked here, since the hypercall does not check if SEV live migration
> is enabled. Similarly, the MSR for enabling it is always available,
> even if userspace didn't ask for the cpuid bit to be set. This should
> not happen. Userspace should be in control of a new hypercall rolling
> out.
> 
> I believe my interpretation last time was that the cpuid bit was
> getting surfaced from the host kernel to host userspace, but I don't
> actually see that in this patch series. Another way to ask this
> question would be "How does userspace know the kernel they are on has
> this patch series?". It needs some way of checking whether or not the
> kernel underneath it supports SEV live migration. Technically, I think
> userspace could call get_cpuid, set_cpuid (with the same values), and
> then get_cpuid again, and it would be able to infer by checking the
> SEV LM feature flag in the KVM leaf. This seems a bit kludgy. Checking
> support should be easy.
> 
> An additional question is "how does userspace choose whether live
> migration is advertised to the guest"? I believe userspace's desire
> for a particular value of the paravirt feature flag in CPUID get's
> overridden when they call set cpuid, since the feature flag is set in
> svm_vcpu_after_set_cpuid regardless of what userspace asks for.
> Userspace should have a choice in the matter.
> 

To summarize, KVM (host) enables SEV live migration feature as
following:

static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
{
...
        /*
         * If SEV guest then enable the Live migration feature.
         */
        if (sev_guest(vcpu->kvm)) {
                struct kvm_cpuid_entry2 *best;

                best = kvm_find_cpuid_entry(vcpu, KVM_CPUID_FEATURES, 0);
                if (!best)
                        return;

                best->eax |= (1 << KVM_FEATURE_SEV_LIVE_MIGRATION);
        }

...
...

Later userspace can call cpuid(KVM_CPUID_FEATURES) and get the cpuid data
and override it, for example, this is how Qemu userspace code currently
fixups/overrides the KVM reported CPUID features : 

target/i386/kvm/kvm.c:

uint32_t kvm_arch_get_supported_cpuid(KVMState *s, uint32_t function,
                                      uint32_t index, int reg)
{
...
...

  cpuid = get_supported_cpuid(s);

  struct kvm_cpuid_entry2 *entry = cpuid_find_entry(cpuid, function, index);
  if (entry) {
      ret = cpuid_entry_get_reg(entry, reg);
  }
    
  /* Fixups for the data returned by KVM, below */

  ...
  ...

  } else if (function == KVM_CPUID_FEATURES && reg == R_EAX) {
        /* kvm_pv_unhalt is reported by GET_SUPPORTED_CPUID, but it can't
         * be enabled without the in-kernel irqchip
         */
        if (!kvm_irqchip_in_kernel()) {
            ret &= ~(1U << KVM_FEATURE_PV_UNHALT);
        }
        if (kvm_irqchip_is_split()) {
            ret |= 1U << KVM_FEATURE_MSI_EXT_DEST_ID;
        }
    } else if (function == KVM_CPUID_FEATURES && reg == R_EDX) {
        ret |= 1U << KVM_HINTS_REALTIME;
    }
    
    return ret;

So you can use a similar approach to override
KVM_FEATURE_SEV_LIVE_MIGRATION feature.

Thanks,
Ashish

> Looking at similar paravirt-y features, there's precedent for another
> way of doing this (may be preferred over CHECK_EXTENSION/ENABLE_CAP?):
> this could call guest_pv_has before running the hypercall. The feature
> (KVM_FEATURE_SEV_LIVE_MIGRATION) would then need to be exposed with
> the other paravirt features in __do_cpuid_func. The function
> guest_pv_has would represent if userspace has decided to expose SEV
> live migration to the guest, and the sev->live_migration_enabled would
> indicate if the guest responded affirmatively to the CPUID bit.
> 
> The downside of using guest_pv_has is that, if pv enforcement is
> disabled, guest_pv_has will always return true, which seems a bit odd
> for a non-SEV guest. This isn't a deal breaker, but seems a bit odd
> for say, a guest that isn't even running SEV. Using CHECK_EXTENSION
> and ENABLE_CAP sidestep that. I'm also not certain I would call this a
> paravirt feature.
> 
> > And these were your review comments on the above :
> > I see I misunderstood how the CPUID bits get passed
> > through: usermode can still override them. Forgot about the back and
> > forth for CPUID with usermode.
> >
> > So as you mentioned, userspace can still override these and it gets a
> > chance to decide whether or not this should be enabled.
> >
> > Thanks,
> > Ashish
> 
> 
> Thanks,
> Steve

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