Re: [PATCH 5/5] qom: Make container_get() strict to always walk or return container

[Paolo Bonzini <pbonzini@redhat.com>]

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Il mar 19 nov 2024, 22:43 Peter Xu <peterx@redhat.com> ha scritto:

> > The easiest way to check is probably to print the type of every
> successful
> > object_dynamic_cast and object_class_dynamic_cast. I suspect the result
> > will be virtio-blk-device and/or scsi-hd, but maybe those already do an
> > unsafe cast (pointer type cast) instead of object_dynamic_cast.
>
> Yes, it sounds more reasonable to me to optimize specific call sites so far
> rather than provides something generic.

Though it could still be a
> generic API so that devices can opt-in.


One of the things that I am excited about for Rust is checking at compile
time whether a cast is to a superclass, which makes it safe automatically.

> I can give it some measurement if there is, otherwise I'm
> > > guessing whatever changes could fall into the noise.
> >
> >
> > Yes, probably. At most you can identify if there any heavy places out of
> > the 34000 calls, and see if they can use an unsafe cast.
>
> I can still trivially do this.
>
> I traced qemu using bpf


Nice! I want to know more. :))
A

> and interestingly in my case close to half (over
> 10000+) of the calls are about ahci_irq_lower() from different higher level
> stack (yeah I used IDE in my setup.. with a split irqchi..), where it has:
>
>     PCIDevice *pci_dev = (PCIDevice *)
> object_dynamic_cast(OBJECT(dev_state),
>
>  TYPE_PCI_DEVICE);
>
> So IIUC that can be open to a unsafe cast too


Hmm no it can't because there's also sysbus AHCI. The fix would be to add
an AHCIClass and make irq toggling into a method there

but considering IDE is ODD FIXES stage, I'm not sure if I should send a
> patch at all.  However I copied John regardless.
>

Well, MAINTAINERS only says the kind of work that the maintainer is doing,
you can always do more. However it seems like not a small amount, so maybe
adding a comment is enough if somebody else wants to do it?

Paolo


> Thanks,
>
> --
> Peter Xu
>
>

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