On Fri, Oct 14, 2016 at 11:06:55AM +0100, Andrew Cooper wrote:
> Contrary to the description in the software manuals, in Long Mode, attempts to
> load %cs check that D is not set in combination with L before the present flag
> is checked.
>
> This can be observed because the L/D check fails with #GP before the presence
> check failes with #NP
CC-ing Paul and Sherry.
Perhaps the SDMs should mention this as well?
>
> Signed-off-by: Andrew Cooper
> ---
> CC: Jan Beulich
> ---
> xen/arch/x86/x86_emulate/x86_emulate.c | 13 -
> 1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c
> b/xen/arch/x86/x86_emulate/x86_emulate.c
> index 793ce30..b23cd99 100644
> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> @@ -1405,6 +1405,14 @@ protmode_load_seg(
> /* Non-conforming segment: check RPL and DPL against CPL. */
> : rpl > cpl || dpl != cpl )
> goto raise_exn;
> +/*
> + * 64-bit code segments (L bit set) must have D bit clear.
> + * Experimentally in long mode, the L and D bits are checked before
> + * the Present bit.
> + */
> +if ( in_longmode(ctxt, ops) &&
> + (desc.b & (1 << 21)) && (desc.b & (1 << 22)) )
> +goto raise_exn;
> sel = (sel ^ rpl) | cpl;
> break;
> case x86_seg_ss:
> @@ -1444,11 +1452,6 @@ protmode_load_seg(
> goto raise_exn;
> }
>
> -/* 64-bit code segments (L bit set) must have D bit clear. */
> -if ( seg == x86_seg_cs && in_longmode(ctxt, ops) &&
> - (desc.b & (1 << 21)) && (desc.b & (1 << 22)) )
> -goto raise_exn;
> -
> /* Ensure Accessed flag is set. */
> if ( a_flag && !(desc.b & a_flag) )
> {
> --
> 2.1.4
>
>
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