Re: [Xen-devel] [PATCH 13/25] argo: implement the register op

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.12.18 at 06:29, <christopher.w.clark@xxxxxxxxx> wrote:
> On Wed, Dec 12, 2018 at 1:48 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>
>> > +static int
>> > +argo_find_ring_mfns(struct domain *d, struct argo_ring_info *ring_info,
>> > +                    uint32_t npage, XEN_GUEST_HANDLE_PARAM(argo_pfn_t) 
>> > pfn_hnd,
>> > +                    uint32_t len)
>> > +{
>> > +    int i;
>> > +    int ret = 0;
>> > +
>> > +    if ( (npage << PAGE_SHIFT) < len )
>> > +        return -EINVAL;
>> > +
>> > +    if ( ring_info->mfns )
>> > +    {
>> > +        /*
>> > +         * Ring already existed. Check if it's the same ring,
>> > +         * i.e. same number of pages and all translated gpfns still
>> > +         * translating to the same mfns
>> > +         */
>>
>> This comment makes me wonder whether the translations are
>> permitted to change at other times. If so I'm not sure what
>> value verification here has. If not, this probably would want to
>> be debugging-only code.
> 
> My understanding is that the gfn->mfn translation is not necessarily stable
> across entry and exit from host power state S4, suspend to disk.

How would that be? It's not stable across guest migration (or
its non-live save/restore equivalent), but how would things
change across S3? And there's no support for S4 (and I can't
see it appearing any time soon).

>> > +static struct argo_ring_info *
>> > +argo_ring_find_info(const struct domain *d, const struct argo_ring_id *id)
>> > +{
>> > +    uint16_t hash;
>> > +    struct hlist_node *node;
>>
>> const?
> 
> I couldn't determine exactly what you were pointing towards with this one.
> I've applied 'const' in a lot further place in the next version; please
> let me know if I've missed where you intended.

This is a pretty general rule: const should be applied to pointer
target types whenever no modification is intended, to make
this read-only aspect very obvious (and force people to think
twice if they alter such a property).

>> > +    uint64_t dst_domain_cookie = 0;
>> > +
>> > +    if ( !(guest_handle_is_aligned(ring_hnd, ~PAGE_MASK)) )
>> > +        return -EINVAL;
>>
>> Why? You don't store the handle for later use (and you shouldn't).
>> If there really is a need for a full page's worth of memory, it
>> would better be passed in as GFN.
> 
> I've added this comment for this behaviour in v2:
> 
> +    /*
> +     * Verify the alignment of the ring data structure supplied with the
> +     * understanding that the ring handle supplied points to the same memory 
> as
> +     * the first entry in the array of pages provided via pg_descr_hnd, where
> +     * the head of the ring will reside.
> +     * See argo_update_tx_ptr where the location of the tx_ptr is accessed 
> at a
> +     * fixed offset from head of the first page in the mfn array.
> +     */

Well, this then suggests that you don't want to verify alignment,
but instead you want to verify addresses match.

>> > @@ -253,6 +723,34 @@ do_argo_message_op(int cmd, 
>> > XEN_GUEST_HANDLE_PARAM(void) arg1,
>> >
>> >      switch (cmd)
>> >      {
>> > +    case ARGO_MESSAGE_OP_register_ring:
>> > +    {
>> > +        XEN_GUEST_HANDLE_PARAM(argo_ring_t) ring_hnd =
>> > +            guest_handle_cast(arg1, argo_ring_t);
>> > +        XEN_GUEST_HANDLE_PARAM(argo_pfn_t) pfn_hnd =
>> > +            guest_handle_cast(arg2, argo_pfn_t);
>> > +        uint32_t npage = arg3;
>> > +        bool fail_exist = arg4 & ARGO_REGISTER_FLAG_FAIL_EXIST;
>> > +
>> > +        if ( unlikely(!guest_handle_okay(ring_hnd, 1)) )
>> > +            break;
>>
>> I don't understand the need for this and ...
>>
>> > +        if ( unlikely(npage > (ARGO_MAX_RING_SIZE >> PAGE_SHIFT)) )
>> > +        {
>> > +            rc = -EINVAL;
>> > +            break;
>> > +        }
>> > +        if ( unlikely(!guest_handle_okay(pfn_hnd, npage)) )
>> > +            break;
>>
>> ... perhaps also this, when you use copy_from_guest() upon access.
> 
> This is the one piece of feedback on version 1 of this series that I haven't
> taken the time to address yet. The code is evidently safe, with only a 
> possible
> performance decrease a concern, so I'd like to study it further before 
> removing
> any of the checks rather than delay posting version two of this series.

Hmm, re-posting without all comments addressed is not ideal.
It means extra work for the reviewers (unless you've clearly
marked respective code fragments with some sort of TBD
comment).

Jan


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