Re: [Xen-devel] [PATCH] x86/HVM: don't retain emulated insn cache when exiting back to guest

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 05 December 2017 16:14
> To: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> Cc: Julien Grall <julien.grall@xxxxxxx>; Andrew Cooper
> <Andrew.Cooper3@xxxxxxxxxx>; Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Subject: [PATCH] x86/HVM: don't retain emulated insn cache when exiting
> back to guest
> 
> vio->mmio_retry is being set when a repeated string insn is being split
> up. In that case we'll exit to the guest, expecting immediate re-entry.
> Interruptions, however, may be serviced by the guest before re-entry
> from the repeated string insn. Any emulation needed in the course of
> handling the interruption must not fetch from the internally maintained
> cache.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -2109,20 +2109,22 @@ static int _hvm_emulate_one(struct hvm_e
> 
>      vio->mmio_retry = 0;
> 
> -    rc = x86_emulate(&hvmemul_ctxt->ctxt, ops);
> -
> -    if ( rc == X86EMUL_OKAY && vio->mmio_retry )
> -        rc = X86EMUL_RETRY;
> -    if ( rc != X86EMUL_RETRY )
> +    switch ( rc = x86_emulate(&hvmemul_ctxt->ctxt, ops) )
>      {
> +    case X86EMUL_OKAY:
> +        if ( vio->mmio_retry )
> +            rc = X86EMUL_RETRY;
> +        /* fall through */
> +    default:
>          vio->mmio_cache_count = 0;
>          vio->mmio_insn_bytes = 0;
> -    }
> -    else
> -    {
> +        break;
> +
> +    case X86EMUL_RETRY:
>          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;

So, we have two distinct cases when X86EMUL_RETRY will be returned: the former 
when we do want to return to guest part way through a rep operation, and 
another when an MMIO has been sent for external emulation and we are expecting 
a completion. The code looks correct so...

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

...but I wonder there should be two distinct return codes for these two cases.

>      }
> 
>      if ( hvmemul_ctxt->ctxt.retire.singlestep )
> 
> 


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