Re: [PATCH 12/26] KVM: track whether guest state is encrypted

[Xiaoyao Li]

On 3/23/2024 2:11 AM, Paolo Bonzini wrote:
> So far, KVM has allowed KVM_GET/SET_* ioctls to execute even if the
> guest state is encrypted, in which case they do nothing.  For the new
> API using VM types, instead, the ioctls will fail which is a safer and
> more robust approach.
>
> The new API will be the only one available for SEV-SNP and TDX, but it
> is also usable for SEV and SEV-ES.  In preparation for that, require
> architecture-specific KVM code to communicate the point at which guest
> state is protected (which must be after kvm_cpu_synchronize_post_init(),
> though that might change in the future in order to suppor migration).
>  From that point, skip reading registers so that cpu->vcpu_dirty is
> never true: if it ever becomes true, kvm_arch_put_registers() will
> fail miserably.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>   include/sysemu/kvm.h     |  2 ++
>   include/sysemu/kvm_int.h |  1 +
>   accel/kvm/kvm-all.c      | 14 ++++++++++++--
>   target/i386/sev.c        |  1 +
>   4 files changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/include/sysemu/kvm.h b/include/sysemu/kvm.h
> index fad9a7e8ff3..302e8f6f1e5 100644
> --- a/include/sysemu/kvm.h
> +++ b/include/sysemu/kvm.h
> @@ -539,6 +539,8 @@ bool kvm_dirty_ring_enabled(void);
>   
>   uint32_t kvm_dirty_ring_size(void);
>   
> +void kvm_mark_guest_state_protected(void);
> +
>   /**
>    * kvm_hwpoisoned_mem - indicate if there is any hwpoisoned page
>    * reported for the VM.
> diff --git a/include/sysemu/kvm_int.h b/include/sysemu/kvm_int.h
> index 882e37e12c5..3496be7997a 100644
> --- a/include/sysemu/kvm_int.h
> +++ b/include/sysemu/kvm_int.h
> @@ -87,6 +87,7 @@ struct KVMState
>       bool kernel_irqchip_required;
>       OnOffAuto kernel_irqchip_split;
>       bool sync_mmu;
> +    bool guest_state_protected;
>       uint64_t manual_dirty_log_protect;
>       /* The man page (and posix) say ioctl numbers are signed int, but
>        * they're not.  Linux, glibc and *BSD all treat ioctl numbers as
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index a8cecd040eb..05fa3533c66 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -2698,7 +2698,7 @@ bool kvm_cpu_check_are_resettable(void)
>   
>   static void do_kvm_cpu_synchronize_state(CPUState *cpu, run_on_cpu_data arg)
>   {
> -    if (!cpu->vcpu_dirty) {
> +    if (!cpu->vcpu_dirty && !kvm_state->guest_state_protected) {
>           int ret = kvm_arch_get_registers(cpu);
>           if (ret) {
>               error_report("Failed to get registers: %s", strerror(-ret));
> @@ -2712,7 +2712,7 @@ static void do_kvm_cpu_synchronize_state(CPUState *cpu, 
> run_on_cpu_data arg)
>   
>   void kvm_cpu_synchronize_state(CPUState *cpu)
>   {
> -    if (!cpu->vcpu_dirty) {
> +    if (!cpu->vcpu_dirty && !kvm_state->guest_state_protected) {
>           run_on_cpu(cpu, do_kvm_cpu_synchronize_state, RUN_ON_CPU_NULL);
>       }
>   }
> @@ -2747,6 +2747,11 @@ static void do_kvm_cpu_synchronize_post_init(CPUState 
> *cpu, run_on_cpu_data arg)
>   
>   void kvm_cpu_synchronize_post_init(CPUState *cpu)
>   {
> +    /*
> +     * This runs before the machine_init_done notifiers, and is the last
> +     * opportunity to synchronize the state of confidential guests.
> +     */ > +    assert(!kvm_state->guest_state_protected);

So, this requires confidential guests to call 
kvm_mark_guest_state_protected() in its machine_init_done notifier callback?

But for TDX, the guest_state is protected at the beginning, not some 
time later when machine_init_done.

>       run_on_cpu(cpu, do_kvm_cpu_synchronize_post_init, RUN_ON_CPU_NULL);
>   }
>   
> @@ -4094,3 +4099,8 @@ void query_stats_schemas_cb(StatsSchemaList **result, Error **errp)
>           query_stats_schema_vcpu(first_cpu, &stats_args);
>       }
>   }
> +
> +void kvm_mark_guest_state_protected(void)
> +{
> +    kvm_state->guest_state_protected = true;
> +}
> diff --git a/target/i386/sev.c b/target/i386/sev.c
> index b8f79d34d19..c49a8fd55eb 100644
> --- a/target/i386/sev.c
> +++ b/target/i386/sev.c
> @@ -755,6 +755,7 @@ sev_launch_get_measure(Notifier *notifier, void *unused)
>           if (ret) {
>               exit(1);
>           }
> +        kvm_mark_guest_state_protected();
>       }
>   
>       /* query the measurement blob length */