linux-kernel - Re: [PATCH v5 06/17] virt: acrn: Introduce VM management interfaces

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Date:   Thu, 5 Nov 2020 20:48:22 +0800
From:   Shuo A Liu <shuo.a.liu@...el.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Yu Wang <yu1.wang@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Zhi Wang <zhi.a.wang@...el.com>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>
Subject: Re: [PATCH v5 06/17] virt: acrn: Introduce VM management interfaces

On Thu  5.Nov'20 at 10:16:45 +0100, Greg Kroah-Hartman wrote:
>On Thu, Nov 05, 2020 at 05:02:00PM +0800, Shuo A Liu wrote:
>> On Thu  5.Nov'20 at  9:26:39 +0100, Greg Kroah-Hartman wrote:
>> > On Thu, Nov 05, 2020 at 03:35:45PM +0800, Shuo A Liu wrote:
>> > > On Thu  5.Nov'20 at  7:29:07 +0100, Greg Kroah-Hartman wrote:
>> > > > On Thu, Nov 05, 2020 at 11:10:29AM +0800, Shuo A Liu wrote:
>> > > > > On Wed  4.Nov'20 at 20:02:35 +0100, Greg Kroah-Hartman wrote:
>> > > > > > On Mon, Oct 19, 2020 at 02:17:52PM +0800, shuo.a.liu@...el.com wrote:
>> > > > > > > --- /dev/null
>> > > > > > > +++ b/include/uapi/linux/acrn.h
>> > > > > > > @@ -0,0 +1,56 @@
>> > > > > > > +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
>> > > > > > > +/*
>> > > > > > > + * Userspace interface for /dev/acrn_hsm - ACRN Hypervisor Service Module
>> > > > > > > + *
>> > > > > > > + * This file can be used by applications that need to communicate with the HSM
>> > > > > > > + * via the ioctl interface.
>> > > > > > > + */
>> > > > > > > +
>> > > > > > > +#ifndef _UAPI_ACRN_H
>> > > > > > > +#define _UAPI_ACRN_H
>> > > > > > > +
>> > > > > > > +#include <linux/types.h>
>> > > > > > > +
>> > > > > > > +/**
>> > > > > > > + * struct acrn_vm_creation - Info to create a User VM
>> > > > > > > + * @vmid:		User VM ID returned from the hypervisor
>> > > > > > > + * @reserved0:		Reserved
>> > > > > > > + * @vcpu_num:		Number of vCPU in the VM. Return from hypervisor.
>> > > > > > > + * @reserved1:		Reserved
>> > > > > > > + * @uuid:		UUID of the VM. Pass to hypervisor directly.
>> > > > > > > + * @vm_flag:		Flag of the VM creating. Pass to hypervisor directly.
>> > > > > > > + * @ioreq_buf:		Service VM GPA of I/O request buffer. Pass to
>> > > > > > > + *			hypervisor directly.
>> > > > > > > + * @cpu_affinity:	CPU affinity of the VM. Pass to hypervisor directly.
>> > > > > > > + * @reserved2:		Reserved
>> > > > > >
>> > > > > > Reserved and must be 0?
>> > > > >
>> > > > > Not a must.
>> > > >
>> > > > That's guaranteed to come back and bite you in the end.
>> > >
>> > > OK. I can fill them with zero before passing them to hypervisor.
>> > >
>> > > > You all have read the "how to write a good api" document, right?
>> > >
>> > > Is it Documentation/driver-api/ioctl.rst? Or i missed..
>> >
>> > That's one good document, but no, not what I was referring to.  I was
>> > thinking of Documentation/process/adding-syscalls.rst, which is what you
>> > are doing here implicitly with these new ioctls (every ioctl is a brand
>> > new syscall.)
>>
>> I will read it as well. Thanks.
>>
>> >
>> > > > > > What are they reserved for?
>> > > > > >
>> > > > > > Same for all of the reserved fields, why?
>> > > > >
>> > > > > Some reserved fields are to map layout in the hypervisor side, others
>> > > > > are for future use.
>> > > >
>> > > > ioctls should not have these, again, please read the documentation.  If
>> > > > you need something new in the future, just make a new ioctl.
>> > >
>> > > OK. I will remove some reserved fields for scalability.
>> >
>> > "scalability" should have nothing to do with any of this, right?  What
>> > am I missing?
>>
>> Sorry, i meant reserved fields for future use.
>
>Again, this is not how you do that at all.  If you need something "in
>the future", create it then.  What you are doing here ensures that you
>will never be able to do it then either, so don't even pretend :)
>
>Read the syscall document for why this is the case.

Alright. I will remove that type of reserved fields. Thank you.

>
>> > > Though i can
>> > > keep some reserved fields for alignment (and to keep same data structure
>> > > layout with the hypervisor), right?
>> > > Documentation/driver-api/ioctl.rst says that explicit reserved fields
>> > > could be used.
>> >
>> > If you need alignment, yes, that is fine, but that's not what you are
>> > saying these are for.  And if you need alignment, why not move things
>> > around so they are properly aligned.
>> >
>> > And this structure has nothing to do with the hypervisor structure,
>> > that's a internal-kernel structure, not a userspace-visable thing if you
>> > are doing things correctly.
>>
>> It's the same structure with the one in hypervisor. HSM driver
>> doesn't maintain the VM much, it just pass the data for VM creation from
>> userspace to hypervisor.
>
>That sounds ripe for abuse, good luck!

The hypervisor will do the sanity check. In this case, HSM driver can be
kept simple.

Thanks
shuo