Re: [Xen-devel] [PATCH] x86/HVM: never retain emulated insn cache when exiting back to guest

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 20 April 2018 15:19
> To: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Paul Durrant
> <Paul.Durrant@xxxxxxxxxx>; Juergen Gross <jgross@xxxxxxxx>
> Subject: [PATCH] x86/HVM: never retain emulated insn cache when exiting
> back to guest
> 
> Commit 5fcb26e69e ("x86/HVM: don't retain emulated insn cache when
> exiting back to guest") didn't go quite far enough: The insn emulator
> may itself decide to return X86EMUL_RETRY (currently for certain
> CMPXCHG failures and AVX2 gather insns), in which case we'd also exit
> back to guest context. Tie the caching to whether we have an I/O
> completion pending, instead of x86_emulate()'s return value.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Paul Durrant <paul.durrant@xxxxxxxxxx>

> 
> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -2266,22 +2266,20 @@ static int _hvm_emulate_one(struct hvm_e
> 
>      vio->mmio_retry = 0;
> 
> -    switch ( rc = x86_emulate(&hvmemul_ctxt->ctxt, ops) )
> +    rc = x86_emulate(&hvmemul_ctxt->ctxt, ops);
> +    if ( rc == X86EMUL_OKAY && vio->mmio_retry )
> +        rc = X86EMUL_RETRY;
> +
> +    if ( !hvm_vcpu_io_need_completion(vio) )
>      {
> -    case X86EMUL_OKAY:
> -        if ( vio->mmio_retry )
> -            rc = X86EMUL_RETRY;
> -        /* fall through */
> -    default:
>          vio->mmio_cache_count = 0;
>          vio->mmio_insn_bytes = 0;
> -        break;
> -
> -    case X86EMUL_RETRY:
> +    }
> +    else
> +    {
>          BUILD_BUG_ON(sizeof(vio->mmio_insn) < sizeof(hvmemul_ctxt-
> >insn_buf));
>          vio->mmio_insn_bytes = hvmemul_ctxt->insn_buf_bytes;
>          memcpy(vio->mmio_insn, hvmemul_ctxt->insn_buf, vio-
> >mmio_insn_bytes);
> -        break;
>      }
> 
>      if ( hvmemul_ctxt->ctxt.retire.singlestep )
> 
> 


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