Re: [PATCH] xen: Don't cast away const-ness in vcpu_show_registers()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 05/03/2025 7:53 am, Jan Beulich wrote:
> On 03.03.2025 17:52, Andrew Cooper wrote:
>> On 26/02/2025 7:33 am, Jan Beulich wrote:
>>> On 26.02.2025 00:02, Andrew Cooper wrote:
>>>> The final hunk is `(struct vcpu *)v` in disguise, expressed using a runtime
>>>> pointer chase through memory and a technicality of the C type system to 
>>>> work
>>>> around the fact that get_hvm_registers() strictly requires a mutable 
>>>> pointer.
>>>>
>>>> For anyone interested, this is one reason why C cannot optimise any reads
>>>> across sequence points, even for a function purporting to take a const 
>>>> object.
>>>>
>>>> Anyway, have the function correctly state that it needs a mutable vcpu.  
>>>> All
>>>> callers have a mutable vCPU to hand, and it removes the runtime pointer 
>>>> chase
>>>> in x86.
>>>>
>>>> Make one style adjustment in ARM while adjusting the parameter type.
>>>>
>>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>> ---
>>>> CC: Anthony PERARD <anthony.perard@xxxxxxxxxx>
>>>> CC: Michal Orzel <michal.orzel@xxxxxxx>
>>>> CC: Jan Beulich <jbeulich@xxxxxxxx>
>>>> CC: Julien Grall <julien@xxxxxxx>
>>>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>>> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>>>> CC: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
>>>> CC: Bertrand Marquis <bertrand.marquis@xxxxxxx>
>>>>
>>>> RISC-V and PPC don't have this helper yet, not even in stub form.
>>>>
>>>> I expect there will be one objection to this patch.  Since the last time we
>>>> fought over this, speculative vulnerabilities have demonstrated how 
>>>> dangerous
>>>> pointer chases are, and this is a violation of Misra Rule 11.8, even if 
>>>> it's
>>>> not reasonable for Eclair to be able to spot and reject it.
>>> On these grounds
>>> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
>> Thanks.
>>
>>> irrespective of the fact that a function of this name and purpose really, 
>>> really
>>> should be taking a pointer-to-const.
>> No - this is a perfect example of why most functions should *not* take
>> pointer-to-const for complex objects.
>>
>> There is no such thing as an actually-const vcpu or domain; they are all
>> mutable.  The reason why x86 needs a strictly-mutable pointer is because
>> it needs to take a spinlock to negotiate for access to a hardware
>> resource to read some of the registers it needs.
>>
>> This is where there is a semantic gap between "logically doesn't modify"
>> and what the C keyword means.
> And hence (in part) why C++ gained "mutable" ages ago.

Sure.  If we were writing in C++, then an internal splinlock being
mutable would be a fine thing.

But we're writing in a language where there is no such concept.
>> Anything except the-most-trivial trivial predates may reasonably need to
>> take a spinlock or some other safety primitive, even just to read
>> information.
>>
>>
>> Because this was gratuitously const in the first place, bad code was put
>> in place of making the prototype match reality.
>>
>> This demonstrates a bigger failing in how code is reviewed and
>> maintained.  It is far too frequent that requests to const things don't
>> even compile.  It is also far too frequent that requests to const things
>> haven't read the full patch series to realise why not.  Both of these
>> are a source of friction during review.
>>
>> But more than that, even if something could technically be const right
>> now, the request to do so forces churn into a future patch to de-const
>> it in order to make a clean change.  And for contributors who aren't
>> comfortable saying a firm no to a maintainer, this turns into a bad hack
>> instead.
>>
>> i.e. requests to const accessors for complexity objects are making Xen
>> worse, not better, and we should stop doing it.
> Okay, let's agree that we don't agree in our perspectives here.

I'm not saying this to be mean.  If C could do something like C++'s
mutable, then this wouldn't be an issue.

But, I have lost count of the number of times I have had to reject
requests of yours to const a pointer, on the basis that it can't
compile.  Your review feedback cost one of my team-members a week trying
to fulfil a const request before asking me for help, and it was another
impossible example.

Of all feedback given by reviewers (it's not only you), requests to
const are the ones that are most often wrong in my experience.  Probably
only ~50% of requests are correct, yet it takes a very seasoned
developer to come back and say "no, that doesn't compile", because
that's really a "I think you're wrong" needing knowledge in a subtle
part of the language.

My request is to all reviewers.  Please take far more care before asking
for const.  There are absolutely cases where it's right, but a false
request is more problematic than it appears at the surface.

~Andrew