Re: [PATCH v3 2/3] uio_pci_generic: add MSI/MSI-X support

[Vlad Zolotarov]

On 10/05/15 13:57, Greg KH wrote:
> On Mon, Oct 05, 2015 at 01:48:39PM +0300, Vlad Zolotarov wrote:
> > On 10/05/15 10:56, Greg KH wrote:
> > > On Mon, Oct 05, 2015 at 10:41:39AM +0300, Vlad Zolotarov wrote:
> > > > > > +struct msix_info {
> > > > > > +	int num_irqs;
> > > > > > +	struct msix_entry *table;
> > > > > > +	struct uio_msix_irq_ctx {
> > > > > > +		struct eventfd_ctx *trigger;	/* MSI-x vector to eventfd */
> > > > > Why are you using eventfd for msi vectors?  What's the reason for
> > > > > needing this?
> > > > A small correction - for MSI-X vectors. There may be only one MSI vector per
> > > > PCI function and if it's used it would use the same interface as a legacy
> > > > INT#x interrupt uses at the moment.
> > > > So, for MSI-X case the reason is that there may be (in most cases there will
> > > > be) more than one interrupt vector. Thus, as I've explained in a PATCH1
> > > > thread we need a way to indicated each of them separately. eventfd seems
> > > > like a good way of doing so. If u have better ideas, pls., share.
> > > You need to document what you are doing here, I don't see any
> > > explaination for using eventfd at all.
> > >
> > > And no, I don't know of any other solution as I don't know what you are
> > > trying to do here (hint, the changelog didn't document it...)
> > >
> > > > > You haven't documented how this api works at all, you are going to have
> > > > > to a lot more work to justify this, as this greatly increases the
> > > > > complexity of the user/kernel api in unknown ways.
> > > > I actually do documented it a bit. Pls., check PATCH3 out.
> > > That provided no information at all about how to use the api.
> > >
> > > If it did, you would see that your api is broken for 32/64bit kernels
> > > and will fall over into nasty pieces the first time you try to use it
> > > there, which means it hasn't been tested at all :(
> > It has been tested of course ;)
> > I tested it only in 64 bit environment however where both kernel and user
> > space applications were compiled on the same machine with the same compiler
> > and it could be that "int" had the same number of bytes both in kernel and
> > in user space application. Therefore it worked perfectly - I patched DPDK to
> > use the new uio_pci_generic MSI-X API to test this and I have verified that
> > all 3 interrupt modes work: MSI-X with SR-IOV VF device in Amazon EC2 guest
> > and INT#x and MSI with a PF device on bare metal server.
> >
> > However I agree using uint32_t for "vec" and "fd" would be much more
> > correct.
> I don't think file descriptors are __u32 on a 64bit arch, are they?

I think they are "int" on all platforms and as far as I know u32 should 
be enough to contain int on any platform.

> And NEVER use the _t types in kernel code,

Never meant it - it was for a user space interface. For a kernel it's 
u32 of course.

>   the namespaces is all wrong
> and it is not applicable for us, sorry.
>
> thanks,
>
> greg k-h

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