Re: [PATCH 2/2] x86/vm_event: Allow returning i-cache for emulation

[Razvan Cojocaru <rcojocaru@bitdefender.com>]

On 09/13/2016 09:12 PM, Tamas K Lengyel wrote:
> When emulating instructions the emulator maintains a small i-cache fetched
> from the guest memory. This patch extends the vm_event interface to allow
> returning this i-cache via the vm_event response instead.
> 
> When responding to a SOFTWARE_BREAKPOINT event (INT3) the monitor subscriber
> normally has to remove the INT3 from memory - singlestep - place back INT3
> to allow the guest to continue execution. This routine however is susceptible
> to a race-condition on multi-vCPU guests. By allowing the subscriber to return
> the i-cache to be used for emulation it can side-step the problem by returning
> a clean buffer without the INT3 present.
> 
> As part of this patch we rename hvm_mem_access_emulate_one to
> hvm_emulate_one_vm_event to better reflect that it is used in various vm_event
> scenarios now, not just in response to mem_access events.
> 
> Signed-off-by: Tamas K Lengyel <tamas.lengyel@zentific.com>
> ---
> Cc: Paul Durrant <paul.durrant@citrix.com>
> Cc: Jan Beulich <jbeulich@suse.com>
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: Jun Nakajima <jun.nakajima@intel.com>
> Cc: Kevin Tian <kevin.tian@intel.com>
> Cc: George Dunlap <george.dunlap@eu.citrix.com>
> Cc: Razvan Cojocaru <rcojocaru@bitdefender.com>
> Cc: Stefano Stabellini <sstabellini@kernel.org>
> Cc: Julien Grall <julien.grall@arm.com>
> 
> Note: This patch only has been compile-tested.
> ---
>  xen/arch/x86/hvm/emulate.c        | 44 ++++++++++++++++++++++++++-------------
>  xen/arch/x86/hvm/hvm.c            |  9 +++++---
>  xen/arch/x86/hvm/vmx/vmx.c        |  1 +
>  xen/arch/x86/vm_event.c           |  9 +++++++-
>  xen/common/vm_event.c             |  1 -
>  xen/include/asm-x86/hvm/emulate.h |  8 ++++---
>  xen/include/asm-x86/vm_event.h    |  3 ++-
>  xen/include/public/vm_event.h     | 16 +++++++++++++-
>  8 files changed, 67 insertions(+), 24 deletions(-)
> 
> diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
> index cc25676..504ed35 100644
> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -76,9 +76,9 @@ static int set_context_data(void *buffer, unsigned int size)
>      if ( curr->arch.vm_event )
>      {
>          unsigned int safe_size =
> -            min(size, curr->arch.vm_event->emul_read_data.size);
> +            min(size, curr->arch.vm_event->emul_read.size);
>  
> -        memcpy(buffer, curr->arch.vm_event->emul_read_data.data, safe_size);
> +        memcpy(buffer, curr->arch.vm_event->emul_read.data, safe_size);
>          memset(buffer + safe_size, 0, size - safe_size);
>          return X86EMUL_OKAY;
>      }
> @@ -827,7 +827,7 @@ static int hvmemul_read(
>      struct hvm_emulate_ctxt *hvmemul_ctxt =
>          container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
>  
> -    if ( unlikely(hvmemul_ctxt->set_context) )
> +    if ( unlikely(hvmemul_ctxt->set_context_data) )
>          return set_context_data(p_data, bytes);
>  
>      return __hvmemul_read(
> @@ -1029,7 +1029,7 @@ static int hvmemul_cmpxchg(
>      struct hvm_emulate_ctxt *hvmemul_ctxt =
>          container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
>  
> -    if ( unlikely(hvmemul_ctxt->set_context) )
> +    if ( unlikely(hvmemul_ctxt->set_context_data) )
>      {
>          int rc = set_context_data(p_new, bytes);
>  
> @@ -1122,7 +1122,7 @@ static int hvmemul_rep_outs(
>      p2m_type_t p2mt;
>      int rc;
>  
> -    if ( unlikely(hvmemul_ctxt->set_context) )
> +    if ( unlikely(hvmemul_ctxt->set_context_data) )
>          return hvmemul_rep_outs_set_context(src_seg, src_offset, dst_port,
>                                              bytes_per_rep, reps, ctxt);
>  
> @@ -1264,7 +1264,7 @@ static int hvmemul_rep_movs(
>      if ( buf == NULL )
>          return X86EMUL_UNHANDLEABLE;
>  
> -    if ( unlikely(hvmemul_ctxt->set_context) )
> +    if ( unlikely(hvmemul_ctxt->set_context_data) )
>      {
>          rc = set_context_data(buf, bytes);
>  
> @@ -1470,7 +1470,7 @@ static int hvmemul_read_io(
>  
>      *val = 0;
>  
> -    if ( unlikely(hvmemul_ctxt->set_context) )
> +    if ( unlikely(hvmemul_ctxt->set_context_data) )
>          return set_context_data(val, bytes);
>  
>      return hvmemul_do_pio_buffer(port, bytes, IOREQ_READ, val);
> @@ -1793,7 +1793,14 @@ static int _hvm_emulate_one(struct hvm_emulate_ctxt *hvmemul_ctxt,
>          pfec |= PFEC_user_mode;
>  
>      hvmemul_ctxt->insn_buf_eip = regs->eip;
> -    if ( !vio->mmio_insn_bytes )
> +
> +    if ( unlikely(hvmemul_ctxt->set_context_insn) && curr->arch.vm_event )
> +    {
> +        hvmemul_ctxt->insn_buf_bytes = sizeof(curr->arch.vm_event->emul_insn);
> +        memcpy(hvmemul_ctxt->insn_buf, &curr->arch.vm_event->emul_insn,
> +               hvmemul_ctxt->insn_buf_bytes);
> +    }
> +    else if ( !vio->mmio_insn_bytes )
>      {
>          hvmemul_ctxt->insn_buf_bytes =
>              hvm_get_insn_bytes(curr, hvmemul_ctxt->insn_buf) ?:
> @@ -1931,7 +1938,7 @@ int hvm_emulate_one_mmio(unsigned long mfn, unsigned long gla)
>      return rc;
>  }
>  
> -void hvm_mem_access_emulate_one(enum emul_kind kind, unsigned int trapnr,
> +void hvm_emulate_one_vm_event(enum emul_kind kind, unsigned int trapnr,
>      unsigned int errcode)
>  {
>      struct hvm_emulate_ctxt ctx = {{ 0 }};
> @@ -1944,11 +1951,19 @@ void hvm_mem_access_emulate_one(enum emul_kind kind, unsigned int trapnr,
>      case EMUL_KIND_NOWRITE:
>          rc = hvm_emulate_one_no_write(&ctx);
>          break;
> -    case EMUL_KIND_SET_CONTEXT:
> -        ctx.set_context = 1;
> -        /* Intentional fall-through. */
> -    default:
> +    case EMUL_KIND_SET_CONTEXT_DATA:
> +        ctx.set_context_data = 1;
> +        rc = hvm_emulate_one(&ctx);
> +        break;
> +    case EMUL_KIND_SET_CONTEXT_INSN:
> +        ctx.set_context_insn = 1;
>          rc = hvm_emulate_one(&ctx);
> +        break;
> +    case EMUL_KIND_NORMAL:
> +        rc = hvm_emulate_one(&ctx);
> +        break;
> +    default:
> +        return;
>      }
>  
>      switch ( rc )
> @@ -1983,7 +1998,8 @@ void hvm_emulate_prepare(
>      hvmemul_ctxt->ctxt.force_writeback = 1;
>      hvmemul_ctxt->seg_reg_accessed = 0;
>      hvmemul_ctxt->seg_reg_dirty = 0;
> -    hvmemul_ctxt->set_context = 0;
> +    hvmemul_ctxt->set_context_data = 0;
> +    hvmemul_ctxt->set_context_insn = 0;
>      hvmemul_get_seg_reg(x86_seg_cs, hvmemul_ctxt);
>      hvmemul_get_seg_reg(x86_seg_ss, hvmemul_ctxt);
>  }
> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> index ca96643..7462794 100644
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -489,13 +489,16 @@ void hvm_do_resume(struct vcpu *v)
>  
>              if ( v->arch.vm_event->emulate_flags &
>                   VM_EVENT_FLAG_SET_EMUL_READ_DATA )
> -                kind = EMUL_KIND_SET_CONTEXT;
> +                kind = EMUL_KIND_SET_CONTEXT_DATA;
>              else if ( v->arch.vm_event->emulate_flags &
>                        VM_EVENT_FLAG_EMULATE_NOWRITE )
>                  kind = EMUL_KIND_NOWRITE;
> +            else if ( v->arch.vm_event->emulate_flags &
> +                 VM_EVENT_FLAG_SET_EMUL_INSN_DATA )
> +                kind = EMUL_KIND_SET_CONTEXT_INSN;
>  
> -            hvm_mem_access_emulate_one(kind, TRAP_invalid_op,
> -                                       HVM_DELIVER_NO_ERROR_CODE);
> +            hvm_emulate_one_vm_event(kind, TRAP_invalid_op,
> +                                     HVM_DELIVER_NO_ERROR_CODE);
>  
>              v->arch.vm_event->emulate_flags = 0;
>          }
> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> index 2759e6f..d214716 100644
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -57,6 +57,7 @@
>  #include <asm/altp2m.h>
>  #include <asm/event.h>
>  #include <asm/monitor.h>
> +#include <asm/vm_event.h>
>  #include <public/arch-x86/cpuid.h>
>  
>  static bool_t __initdata opt_force_ept;
> diff --git a/xen/arch/x86/vm_event.c b/xen/arch/x86/vm_event.c
> index 343b9c8..03beed3 100644
> --- a/xen/arch/x86/vm_event.c
> +++ b/xen/arch/x86/vm_event.c
> @@ -209,11 +209,18 @@ void vm_event_emulate_check(struct vcpu *v, vm_event_response_t *rsp)
>          if ( p2m_mem_access_emulate_check(v, rsp) )
>          {
>              if ( rsp->flags & VM_EVENT_FLAG_SET_EMUL_READ_DATA )
> -                v->arch.vm_event->emul_read_data = rsp->data.emul_read_data;
> +                v->arch.vm_event->emul_read = rsp->data.emul.read;
>  
>              v->arch.vm_event->emulate_flags = rsp->flags;
>          }
>          break;
> +    case VM_EVENT_REASON_SOFTWARE_BREAKPOINT:
> +        if ( rsp->flags & VM_EVENT_FLAG_SET_EMUL_INSN_DATA )
> +        {
> +            v->arch.vm_event->emul_insn = rsp->data.emul.insn;
> +            v->arch.vm_event->emulate_flags = rsp->flags;
> +        }
> +        break;
>      default:
>          break;
>      };
> diff --git a/xen/common/vm_event.c b/xen/common/vm_event.c
> index 907ab40..d8ee7f3 100644
> --- a/xen/common/vm_event.c
> +++ b/xen/common/vm_event.c
> @@ -398,7 +398,6 @@ void vm_event_resume(struct domain *d, struct vm_event_domain *ved)
>           * In some cases the response type needs extra handling, so here
>           * we call the appropriate handlers.
>           */
> -
>          /* Check flags which apply only when the vCPU is paused */
>          if ( atomic_read(&v->vm_event_pause_count) )
>          {
> diff --git a/xen/include/asm-x86/hvm/emulate.h b/xen/include/asm-x86/hvm/emulate.h
> index 3aabcbe..b52f99e 100644
> --- a/xen/include/asm-x86/hvm/emulate.h
> +++ b/xen/include/asm-x86/hvm/emulate.h
> @@ -34,20 +34,22 @@ struct hvm_emulate_ctxt {
>  
>      uint32_t intr_shadow;
>  
> -    bool_t set_context;
> +    bool_t set_context_data;
> +    bool_t set_context_insn;
>  };
>  
>  enum emul_kind {
>      EMUL_KIND_NORMAL,
>      EMUL_KIND_NOWRITE,
> -    EMUL_KIND_SET_CONTEXT
> +    EMUL_KIND_SET_CONTEXT_DATA,
> +    EMUL_KIND_SET_CONTEXT_INSN

Since this breaks compilation of existing clients, I think we should
increment some macro to alow for compile-time switching (maybe
VM_EVENT_INTERFACE_VERSION?).


Thanks,
Razvan

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