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SubjectRe: [PATCH v10 12/16] KVM: x86: Introduce new KVM_FEATURE_SEV_LIVE_MIGRATION feature & Custom MSR.
Hello Steve,

On Sat, Feb 06, 2021 at 05:46:17AM +0000, Ashish Kalra wrote:
> 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@amd.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@amd.com> wrote:
> > > > >
> > > > > From: Ashish Kalra <ashish.kalra@amd.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@amd.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.
> >

Actually i did some more analysis of this, and i believe you are right
about the above, feature flag gets set in svm_vcpu_after_set_cpuid.

So please ignore my comments below.

I am still analyzing this further.

Thanks,
Ashish
>
> 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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