Re: [Xen-devel] [PATCH for-4.7] x86/hvm: Correct emulation of invlpg instruction

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.04.16 at 13:18, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 22/04/16 11:30, Jan Beulich wrote:
>>>>> On 22.04.16 at 12:16, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> On 22/04/16 10:57, Jan Beulich wrote:
>>>>>>> On 22.04.16 at 11:48, <Paul.Durrant@xxxxxxxxxx> wrote:
>>>>>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>>>>>> Sent: 22 April 2016 10:31
>>>>>>>>> On 22.04.16 at 10:59, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>>>>> --- a/xen/arch/x86/hvm/emulate.c
>>>>>>> +++ b/xen/arch/x86/hvm/emulate.c
>>>>>>> @@ -1598,8 +1598,27 @@ static int hvmemul_invlpg(
>>>>>>>      rc = hvmemul_virtual_to_linear(
>>>>>>>          seg, offset, 1, &reps, hvm_access_none, hvmemul_ctxt, &addr);
>>>>>>>
>>>>>>> -    if ( rc == X86EMUL_OKAY )
>>>>>>> +    switch ( rc )
>>>>>>> +    {
>>>>>>> +    case X86EMUL_OKAY:
>>>>>>>          hvm_funcs.invlpg_intercept(addr);
>>>>>>> +        break;
>>>>>>> +
>>>>>>> +    case X86EMUL_EXCEPTION:
>>>>>>> +        ASSERT(hvmemul_ctxt->trap.vector == TRAP_gp_fault);
>>>>>>> +        /*
>>>>>>> +         * `invlpg` and `invlpga` are specified to be NOPs when issued 
>>>>>>> on a
>>>>>>> +         * non-canonical address.  hvmemul_virtual_to_linear() latches 
>>>>>>> a #GP
>>>>>>> +         * which is the useful behaviour for most of its callers.
>>>>>> Here and in the description I'd prefer you to not exclusively refer
>>>>>> to non-canonical addresses - segment limit violations in 32-bit or
>>>>>> compatibility modes are affected as much.
>>>>> ...in which case squashing the #GP would be incorrect, right?
>>>> No, not according to the SDM.
>>> I should check and only squash a #GP(0)
>>>
>>> #GP(sel) or #SS(sel) should not be squashed.
>> Which also can't happen here (these only occur when selectors
>> get loaded via some the various mechanisms allowing that).
> 
> #GP(sel) or #SS(sel) also occur for a memory access which causes a
> segment limit violation.  (SDM Vol 3, 5.3 "Limit Checking")

I can't find any mention of error codes in this section at all. Not
sure which version of it you're looking at, mine is 057US. And
even if it said so, I'd be 99.999% certain this is in error, as all
instruction pages always say #GP(0) and #SS(0) for limit
violations.

Jan


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