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

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Date:	Mon, 5 Oct 2015 11:57:15 +0100
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Vlad Zolotarov <vladz@...udius-systems.com>
Cc:	linux-kernel@...r.kernel.org, mst@...hat.com, hjk@...sjkoch.de,
	corbet@....net, bruce.richardson@...el.com,
	avi@...udius-systems.com, gleb@...udius-systems.com,
	stephen@...workplumber.org, alexander.duyck@...il.com
Subject: Re: [PATCH v3 2/3] uio_pci_generic: add MSI/MSI-X support

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?

And NEVER use the _t types in kernel code, the namespaces is all wrong
and it is not applicable for us, sorry.

thanks,

greg k-h
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