RE: [PATCH v2 for-4.14 1/2] x86/mem_sharing: block interrupt injection for forks

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Lengyel, Tamas <tamas.lengyel@xxxxxxxxx>
> Sent: Saturday, May 23, 2020 12:34 AM
> 
> When running shallow forks without device models it may be undesirable for
> Xen

what is shallow forks? and why interrupt injection is not desired without
device model? If it means just without Qemu thing, you still get local APIC
interrupts such as timers, PMI, etc.

> to inject interrupts. With Windows forks we have observed the kernel going
> into
> infinite loops when trying to process such interrupts, likely because it
> attempts

what is the relationship between shallow forks and windows forks then?

> to interact with devices that are not responding without QEMU running. By
> disabling interrupt injection the fuzzer can exercise the target code without
> interference.

what is the fuzzer?

> 
> Forks & memory sharing are only available on Intel CPUs so this only applies
> to vmx.

I feel lots of background is missing thus difficult to judge whether below 
change
is desired...

> 
> Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>
> ---
> v2: prohibit => block
>     minor style adjustments
> ---
>  xen/arch/x86/hvm/vmx/intr.c      | 6 ++++++
>  xen/arch/x86/mm/mem_sharing.c    | 6 +++++-
>  xen/include/asm-x86/hvm/domain.h | 2 ++
>  xen/include/public/memory.h      | 1 +
>  4 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/x86/hvm/vmx/intr.c b/xen/arch/x86/hvm/vmx/intr.c
> index 000e14af49..80bfbb4787 100644
> --- a/xen/arch/x86/hvm/vmx/intr.c
> +++ b/xen/arch/x86/hvm/vmx/intr.c
> @@ -256,6 +256,12 @@ void vmx_intr_assist(void)
>      if ( unlikely(v->arch.vm_event) && v->arch.vm_event->sync_event )
>          return;
> 
> +#ifdef CONFIG_MEM_SHARING
> +    /* Block event injection for VM fork if requested */
> +    if ( unlikely(v->domain->arch.hvm.mem_sharing.block_interrupts) )
> +        return;
> +#endif
> +
>      /* Crank the handle on interrupt state. */
>      pt_vector = pt_update_irq(v);
> 
> diff --git a/xen/arch/x86/mm/mem_sharing.c
> b/xen/arch/x86/mm/mem_sharing.c
> index 7271e5c90b..0c45a8d67e 100644
> --- a/xen/arch/x86/mm/mem_sharing.c
> +++ b/xen/arch/x86/mm/mem_sharing.c
> @@ -2106,7 +2106,8 @@ int
> mem_sharing_memop(XEN_GUEST_HANDLE_PARAM(xen_mem_sharing_op
> _t) arg)
>          rc = -EINVAL;
>          if ( mso.u.fork.pad )
>              goto out;
> -        if ( mso.u.fork.flags & ~XENMEM_FORK_WITH_IOMMU_ALLOWED )
> +        if ( mso.u.fork.flags &
> +             ~(XENMEM_FORK_WITH_IOMMU_ALLOWED |
> XENMEM_FORK_BLOCK_INTERRUPTS) )
>              goto out;
> 
>          rc = rcu_lock_live_remote_domain_by_id(mso.u.fork.parent_domain,
> @@ -2134,6 +2135,9 @@ int
> mem_sharing_memop(XEN_GUEST_HANDLE_PARAM(xen_mem_sharing_op
> _t) arg)
>              rc = hypercall_create_continuation(__HYPERVISOR_memory_op,
>                                                 "lh", XENMEM_sharing_op,
>                                                 arg);
> +        else if ( !rc && (mso.u.fork.flags &
> XENMEM_FORK_BLOCK_INTERRUPTS) )
> +            d->arch.hvm.mem_sharing.block_interrupts = true;
> +
>          rcu_unlock_domain(pd);
>          break;
>      }
> diff --git a/xen/include/asm-x86/hvm/domain.h b/xen/include/asm-
> x86/hvm/domain.h
> index 95fe18cddc..37e494d234 100644
> --- a/xen/include/asm-x86/hvm/domain.h
> +++ b/xen/include/asm-x86/hvm/domain.h
> @@ -74,6 +74,8 @@ struct mem_sharing_domain
>       * to resume the search.
>       */
>      unsigned long next_shared_gfn_to_relinquish;
> +
> +    bool block_interrupts;
>  };
>  #endif
> 
> diff --git a/xen/include/public/memory.h b/xen/include/public/memory.h
> index dbd35305df..1e4959638d 100644
> --- a/xen/include/public/memory.h
> +++ b/xen/include/public/memory.h
> @@ -537,6 +537,7 @@ struct xen_mem_sharing_op {
>          struct mem_sharing_op_fork {      /* OP_FORK */
>              domid_t parent_domain;        /* IN: parent's domain id */
>  #define XENMEM_FORK_WITH_IOMMU_ALLOWED (1u << 0)
> +#define XENMEM_FORK_BLOCK_INTERRUPTS   (1u << 1)
>              uint16_t flags;               /* IN: optional settings */
>              uint32_t pad;                 /* Must be set to 0 */
>          } fork;
> --
> 2.25.1