Re: [Xen-devel] [PATCH 3/4] x86/hvm: Optimise segment accesses in hvmemul_write_segment()

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> Sent: 08 September 2016 15:12
> To: Xen-devel <xen-devel@xxxxxxxxxxxxx>
> Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Jan Beulich
> <JBeulich@xxxxxxxx>; Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Subject: [PATCH 3/4] x86/hvm: Optimise segment accesses in
> hvmemul_write_segment()
> 
> There is no need to read the segment information from VMCS/VMCB and
> cache it, just to clobber the cached content immediately afterwards.
> 
> Write straight into the cache and set the accessed/dirty bits.
> 

Yes, the way the code is now does look somewhat silly.

> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Paul Durrant <paul.durrant@xxxxxxxxxx>

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

> ---
>  xen/arch/x86/hvm/emulate.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
> index 0eb7a4d..e3bfda5 100644
> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -1447,12 +1447,12 @@ static int hvmemul_write_segment(  {
>      struct hvm_emulate_ctxt *hvmemul_ctxt =
>          container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
> -    struct segment_register *sreg = hvmemul_get_seg_reg(seg,
> hvmemul_ctxt);
> 
> -    if ( IS_ERR(sreg) )
> -         return -PTR_ERR(sreg);
> +    if ( seg < 0 || seg >= ARRAY_SIZE(hvmemul_ctxt->seg_reg) )
> +        return X86EMUL_UNHANDLEABLE;
> 
> -    memcpy(sreg, reg, sizeof(struct segment_register));
> +    hvmemul_ctxt->seg_reg[seg] = *reg;
> +    __set_bit(seg, &hvmemul_ctxt->seg_reg_accessed);
>      __set_bit(seg, &hvmemul_ctxt->seg_reg_dirty);
> 
>      return X86EMUL_OKAY;
> --
> 2.1.4


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