Re: [Xen-devel] [PATCH v4 7/7] x86/HVM: reduce scope of pfec in hvm_emulate_init_per_insn()

["Durrant, Paul" <pdurrant@amazon.co.uk>]

> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@lists.xenproject.org> On Behalf Of Jan
> Beulich
> Sent: 31 January 2020 16:46
> To: xen-devel@lists.xenproject.org
> Cc: George Dunlap <George.Dunlap@eu.citrix.com>; Andrew Cooper
> <andrew.cooper3@citrix.com>; Roger Pau Monné <roger.pau@citrix.com>; Wei
> Liu <wl@xen.org>; Paul Durrant <paul@xen.org>
> Subject: [Xen-devel] [PATCH v4 7/7] x86/HVM: reduce scope of pfec in
> hvm_emulate_init_per_insn()
> 
> It needs calculating only in one out of three cases. Re-structure the
> code a little such that the variable truly gets calculated only when we
> don't get any insn bytes from elsewhere, and hence need to (try to)
> fetch them. Also OR in PFEC_insn_fetch right in the initializer.
> 
> While in this mood, restrict addr's scope as well.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Reviewed-by: Paul Durrant <pdurrant@amazon.com>

> ---
> v4: New.
> 
> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -2762,8 +2762,6 @@ void hvm_emulate_init_per_insn(
>      unsigned int insn_bytes)
>  {
>      struct vcpu *curr = current;
> -    unsigned int pfec = PFEC_page_present;
> -    unsigned long addr;
> 
>      hvmemul_ctxt->ctxt.lma = hvm_long_mode_active(curr);
> 
> @@ -2778,14 +2776,23 @@ void hvm_emulate_init_per_insn(
>              hvmemul_ctxt->seg_reg[x86_seg_ss].db ? 32 : 16;
>      }
> 
> -    if ( hvmemul_ctxt->seg_reg[x86_seg_ss].dpl == 3 )
> -        pfec |= PFEC_user_mode;
> -
>      hvmemul_ctxt->insn_buf_eip = hvmemul_ctxt->ctxt.regs->rip;
> -    if ( !insn_bytes )
> +
> +    if ( insn_bytes )
>      {
> +        hvmemul_ctxt->insn_buf_bytes = insn_bytes;
> +        memcpy(hvmemul_ctxt->insn_buf, insn_buf, insn_bytes);
> +    }
> +    else if ( !(hvmemul_ctxt->insn_buf_bytes =
> +                hvm_get_insn_bytes(curr, hvmemul_ctxt->insn_buf)) )
> +    {
> +        unsigned int pfec = PFEC_page_present | PFEC_insn_fetch;
> +        unsigned long addr;
> +
> +        if ( hvmemul_ctxt->seg_reg[x86_seg_ss].dpl == 3 )
> +            pfec |= PFEC_user_mode;
> +
>          hvmemul_ctxt->insn_buf_bytes =
> -            hvm_get_insn_bytes(curr, hvmemul_ctxt->insn_buf) ?:
>              (hvm_virtual_to_linear_addr(x86_seg_cs,
>                                          &hvmemul_ctxt->seg_reg[x86_seg_cs],
>                                          hvmemul_ctxt->insn_buf_eip,
> @@ -2795,15 +2802,9 @@ void hvm_emulate_init_per_insn(
>                                          &addr) &&
>               hvm_copy_from_guest_linear(hvmemul_ctxt->insn_buf, addr,
>                                          sizeof(hvmemul_ctxt->insn_buf),
> -                                        pfec | PFEC_insn_fetch,
> -                                        NULL) == HVMTRANS_okay) ?
> +                                        pfec, NULL) == HVMTRANS_okay) ?
>              sizeof(hvmemul_ctxt->insn_buf) : 0;
>      }
> -    else
> -    {
> -        hvmemul_ctxt->insn_buf_bytes = insn_bytes;
> -        memcpy(hvmemul_ctxt->insn_buf, insn_buf, insn_bytes);
> -    }
>  }
> 
>  void hvm_emulate_writeback(
> 
> 
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