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

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Message-ID: <56125A6C.7030107@scylladb.com>
Date:	Mon, 5 Oct 2015 14:09:32 +0300
From:	Avi Kivity <avi@...lladb.com>
To:	Greg KH <gregkh@...uxfoundation.org>,
	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 10/05/2015 01:57 PM, 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?
>
> And NEVER use the _t types in kernel code, the namespaces is all wrong
> and it is not applicable for us, sorry.

Wasn't the real reason that they aren't defined (or reserved) by C89, 
and therefore could clash with a user identifier, rather than some 
inherent wrongness?

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