Re: [PATCH v2 1/5] xen/common: introduce a new framework for save/restore of 'domain' context

[Jan Beulich <jbeulich@suse.com>]

On 07.05.2020 09:45, Paul Durrant wrote:
>> -----Original Message-----
>> From: Jan Beulich <jbeulich@suse.com>
>> Sent: 07 May 2020 08:40
>> To: paul@xen.org
>> Cc: xen-devel@lists.xenproject.org; 'Paul Durrant' <pdurrant@amazon.com>; 'Andrew Cooper'
>> <andrew.cooper3@citrix.com>; 'George Dunlap' <george.dunlap@citrix.com>; 'Ian Jackson'
>> <ian.jackson@eu.citrix.com>; 'Julien Grall' <julien@xen.org>; 'Stefano Stabellini'
>> <sstabellini@kernel.org>; 'Wei Liu' <wl@xen.org>; 'Volodymyr Babchuk' <Volodymyr_Babchuk@epam.com>;
>> 'Roger Pau Monné' <roger.pau@citrix.com>
>> Subject: Re: [PATCH v2 1/5] xen/common: introduce a new framework for save/restore of 'domain' context
>>
>> On 07.05.2020 09:34, Paul Durrant wrote:
>>>> From: Jan Beulich <jbeulich@suse.com>
>>>> Sent: 07 May 2020 08:22
>>>>
>>>> On 06.05.2020 18:44, Paul Durrant wrote:
>>>>>> From: Jan Beulich <jbeulich@suse.com>
>>>>>> Sent: 29 April 2020 12:02
>>>>>>
>>>>>> On 07.04.2020 19:38, Paul Durrant wrote:
>>>>>>> +int domain_load_begin(struct domain_context *c, unsigned int tc,
>>>>>>> +                      const char *name, const struct vcpu *v, size_t len,
>>>>>>> +                      bool exact)
>>>>>>> +{
>>>>>>> +    if ( c->log )
>>>>>>> +        gdprintk(XENLOG_INFO, "%pv load: %s (%lu)\n", v, name,
>>>>>>> +                 (unsigned long)len);
>>>>>>> +
>>>>>>> +    BUG_ON(tc != c->desc.typecode);
>>>>>>> +    BUG_ON(v->vcpu_id != c->desc.vcpu_id);
>>>>>>> +
>>>>>>> +    if ( (exact && (len != c->desc.length)) ||
>>>>>>> +         (len < c->desc.length) )
>>>>>>> +        return -EINVAL;
>>>>>>
>>>>>> How about
>>>>>>
>>>>>>     if ( exact ? len != c->desc.length
>>>>>>                : len < c->desc.length )
>>>>>>
>>>>>
>>>>> Yes, that doesn't look too bad.
>>>>>
>>>>>> ? I'm also unsure about the < - don't you mean > instead? Too
>>>>>> little data would be compensated by zero padding, but too
>>>>>> much data can't be dealt with. But maybe I'm getting the sense
>>>>>> of len wrong ...
>>>>>
>>>>> I think the < is correct. The caller needs to have at least enough
>>>>> space to accommodate the context record.
>>>>
>>>> But this is load, not save - the caller supplies the data. If
>>>> there's less data than can be fit, it'll be zero-extended. If
>>>> there's too much data, the excess you don't know what to do
>>>> with (it might be okay to tolerate it being all zero).
>>>>
>>>
>>> But this is a callback. The outer load function iterates over
>>> the records calling the appropriate hander for each one. Those
>>> handlers then call this function saying how much data they
>>> expect and whether they want exactly that amount, or whether
>>> they can tolerate less (i.e. zero-extend). Hence
>>> len < c->desc.length is an error, because it means the
>>> descriptor contains more data than the hander knows how to
>>> handle.
>>
>> Oh, I see - "But maybe I'm getting the sense of len wrong ..."
>> then indeed applies.
>>
>> Any thoughts on tolerating the excess data being zero?
>>
> 
> Well the point of the check here is to not tolerate excess data...
> Are you suggesting that it might be a reasonable idea?

Well - it looks to be the obvious counterpart to zero-extending.
I'm not going to assert though that I've thought through all
possible consequences...

Jan