Re: [Xen-devel] [PATCH v5 06/15] VMX/altp2m: add code to support EPTP switching and #VE.

["Sahita, Ravi" <ravi.sahita@xxxxxxxxx>]

>From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>Sent: Thursday, July 16, 2015 2:39 AM
>
>>>> On 16.07.15 at 11:20, <ravi.sahita@xxxxxxxxx> wrote:
>>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>>>Sent: Tuesday, July 14, 2015 6:57 AM
>>>>>> On 14.07.15 at 02:14, <edmund.h.white@xxxxxxxxx> wrote:
>>>> +static bool_t vmx_vcpu_emulate_ve(struct vcpu *v) {
>>>> +    bool_t rc = 0;
>>>> +    ve_info_t *veinfo = gfn_x(vcpu_altp2m(v).veinfo_gfn) !=
>>>> +INVALID_GFN
>>>?
>>>> +        hvm_map_guest_frame_rw(gfn_x(vcpu_altp2m(v).veinfo_gfn), 0) :
>>>> +NULL;
>>>> +
>>>> +    if ( !veinfo )
>>>> +        return 0;
>>>> +
>>>> +    if ( veinfo->semaphore != 0 )
>>>> +        goto out;
>>>> +
>>>> +    rc = 1;
>>>> +
>>>> +    veinfo->exit_reason = EXIT_REASON_EPT_VIOLATION;
>>>> +    veinfo->semaphore = ~0l;
>>>
>>>Isn't semaphore a 32-bit quantity?
>>
>> Yes.
>
>I.e. the l suffix can and should be dropped.
>

Ok.

>>>> +    {
>>>> +        unsigned long idx;
>>>> +
>>>> +        if ( v->arch.hvm_vmx.secondary_exec_control &
>>>> +            SECONDARY_EXEC_ENABLE_VIRT_EXCEPTIONS )
>>>> +            __vmread(EPTP_INDEX, &idx);
>>>> +        else
>>>> +        {
>>>> +            unsigned long eptp;
>>>> +
>>>> +            __vmread(EPT_POINTER, &eptp);
>>>> +
>>>> +            if ( (idx = p2m_find_altp2m_by_eptp(v->domain, eptp)) ==
>>>> +                 INVALID_ALTP2M )
>>>> +            {
>>>> +                gdprintk(XENLOG_ERR, "EPTP not found in alternate p2m
>list\n");
>>>> +                domain_crash(v->domain);
>>>> +            }
>>>> +        }
>>>> +
>>>> +        if ( (uint16_t)idx != vcpu_altp2m(v).p2midx )
>>>
>>>Is this cast really necessary?
>>
>> Yes - The index is 16-bits, this reflects how the field is specified
>> in the vmcs also.
>
>While "yes" answers the question, the explanation you give suggests that the
>answer may be wrong: Can idx indeed have bits set beyond bit 15? Because if
>it can't, the cast is pointless.
>

We were just trying to ensure we matched the hardware behavior (I think there 
was a message George had posted earlier for SVE that asked for that).
Since hardware considers only a 16 bit field we were doing the same.

>>>> +        {
>>>> +            BUG_ON(idx >= MAX_ALTP2M);
>>>> +            atomic_dec(&p2m_get_altp2m(v)->active_vcpus);
>>>> +            vcpu_altp2m(v).p2midx = (uint16_t)idx;
>>>
>>>This one surely isn't (or else the field type is wrong).
>>
>> Again required. idx can't be uint16_t because __vmread() requires
>> unsigned long*, but the index is 16 bits.
>
>But it's a 16-bit VMCS field that you read it from, and hence the upper 48 bits
>are necessarily zero.
>
>Just to re-iterate: Casts are necessary in certain places, yes, but I see them
>used pointlessly or even wrongly more often than not.
>
>Jan

Same approach as above - emulating hardware exactly.
Should we add a comment?

Thanks,
Ravi


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