Re: [Xen-devel] [PATCH v3 13/25] x86emul: adjust_bnd() should check XCR0

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 02/02/18 16:19, Jan Beulich wrote:
>>>> On 02.02.18 at 14:30, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 07/12/17 14:08, Jan Beulich wrote:
>>> Experimentally MPX instructions have been confirmed to behave as NOPs
>>> unless both related XCR0 bits are set to 1. By implication branches
>>> then also don't clear BNDn.
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>
>>> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
>>> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
>>> @@ -2143,12 +2143,16 @@ static bool umip_active(struct x86_emula
>>>  static void adjust_bnd(struct x86_emulate_ctxt *ctxt,
>>>                         const struct x86_emulate_ops *ops, enum vex_pfx pfx)
>>>  {
>>> -    uint64_t bndcfg;
>>> +    uint64_t xcr0, bndcfg;
>>>      int rc;
>>>  
>>>      if ( pfx == vex_f2 || !cpu_has_mpx || !vcpu_has_mpx() )
>>>          return;
>>>  
>>> +    if ( !ops->read_xcr || ops->read_xcr(0, &xcr0, ctxt) != X86EMUL_OKAY ||
>>> +         !(xcr0 & XSTATE_BNDREGS) || !(xcr0 & XSTATE_BNDCSR) )
>> !(xcr0 & (XSTATE_BNDREGS | XSTATE_BNDCSR)) ?
> No, I mean "if either bit is clear", not "if both bits are clear". I think
> we had discussed before that both bits need to be 1 in order for
> bounds checking to actually work.
>
>> Otherwise, Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Please clarify this in light of the above.

Architecturally, they can't be different, which is why the above logic
looks suspicious.

Given that the actual isn't wrong, I won't object, but it does look
wrong to compare them individually.

~Andrew

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