linux-kernel - Re: [PATCH v5 01/18] nitro_enclaves: Add ioctl interface definition

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Date:   Thu, 23 Jul 2020 12:23:56 +0300
From:   "Paraschiv, Andra-Irina" <andraprs@...zon.com>
To:     Greg KH <gregkh@...uxfoundation.org>
CC:     <linux-kernel@...r.kernel.org>,
        Anthony Liguori <aliguori@...zon.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Colm MacCarthaigh <colmmacc@...zon.com>,
        David Duncan <davdunc@...zon.com>,
        Bjoern Doebel <doebel@...zon.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Frank van der Linden <fllinden@...zon.com>,
        Alexander Graf <graf@...zon.de>, Karen Noel <knoel@...hat.com>,
        Martin Pohlack <mpohlack@...zon.de>,
        Matt Wilson <msw@...zon.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Balbir Singh <sblbir@...zon.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        "Stefan Hajnoczi" <stefanha@...hat.com>,
        Stewart Smith <trawets@...zon.com>,
        "Uwe Dannowski" <uwed@...zon.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>, <kvm@...r.kernel.org>,
        <ne-devel-upstream@...zon.com>, Alexander Graf <graf@...zon.com>
Subject: Re: [PATCH v5 01/18] nitro_enclaves: Add ioctl interface definition

On 22/07/2020 12:57, Greg KH wrote:
> On Wed, Jul 22, 2020 at 11:27:29AM +0300, Paraschiv, Andra-Irina wrote:
>>>> +#ifndef _UAPI_LINUX_NITRO_ENCLAVES_H_
>>>> +#define _UAPI_LINUX_NITRO_ENCLAVES_H_
>>>> +
>>>> +#include <linux/types.h>
>>>> +
>>>> +/* Nitro Enclaves (NE) Kernel Driver Interface */
>>>> +
>>>> +#define NE_API_VERSION (1)
>>> Why do you need this version?  It shouldn't be needed, right?
>> The version is used as a way for the user space tooling to sync on the
>> features set provided by the driver e.g. in case an older version of the
>> driver is available on the system and the user space tooling expects a set
>> of features that is not included in that driver version.
> That is guaranteed to get out of sync instantly with different distro
> kernels backporting random things, combined with stable kernel patch
> updates and the like.
>
> Just use the normal api interfaces instead, don't try to "version"
> anything, it will not work, trust us :)
>
> If an ioctl returns -ENOTTY then hey, it's not present and your
> userspace code can handle it that way.

Correct, there could be a variety of kernel versions and user space 
tooling either in the original form, customized or written from scratch. 
And ENOTTY signals an ioctl not available or e.g. EINVAL (or custom 
error) if the parameter field value is not valid within a certain 
version. We have these in place, that's good. :)

However, I was thinking, for example, of an ioctl flow usage where a 
certain order needs to be followed e.g. create a VM, add resources to a 
VM, start a VM.

Let's say, for an use case wrt new features, ioctl A (create a VM) 
succeeds, ioctl B (add memory to the VM) succeeds, ioctl C (add CPU to 
the VM) succeeds and ioctl D (add any other type of resource before 
starting the VM) fails because it is not supported.

Would not need to call ioctl A to C and go through their underneath 
logic to realize ioctl D support is not there and rollback all the 
changes done till then within ioctl A to C logic. Of course, there could 
be ioctl A followed by ioctl D, and would need to rollback ioctl A 
changes, but I shared a more lengthy call chain that can be an option as 
well.

Thanks,
Andra

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