linux-kernel - Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address

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Date:   Fri, 8 Mar 2019 17:15:59 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Jerome Glisse <jglisse@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        peterx@...hat.com, linux-mm@...ck.org, aarcange@...hat.com
Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel
 virtual address


On 2019/3/8 上午11:45, Jerome Glisse wrote:
> On Thu, Mar 07, 2019 at 10:43:12PM -0500, Michael S. Tsirkin wrote:
>> On Thu, Mar 07, 2019 at 10:40:53PM -0500, Jerome Glisse wrote:
>>> On Thu, Mar 07, 2019 at 10:16:00PM -0500, Michael S. Tsirkin wrote:
>>>> On Thu, Mar 07, 2019 at 09:55:39PM -0500, Jerome Glisse wrote:
>>>>> On Thu, Mar 07, 2019 at 09:21:03PM -0500, Michael S. Tsirkin wrote:
>>>>>> On Thu, Mar 07, 2019 at 02:17:20PM -0500, Jerome Glisse wrote:
>>>>>>>> It's because of all these issues that I preferred just accessing
>>>>>>>> userspace memory and handling faults. Unfortunately there does not
>>>>>>>> appear to exist an API that whitelists a specific driver along the lines
>>>>>>>> of "I checked this code for speculative info leaks, don't add barriers
>>>>>>>> on data path please".
>>>>>>> Maybe it would be better to explore adding such helper then remapping
>>>>>>> page into kernel address space ?
>>>>>> I explored it a bit (see e.g. thread around: "__get_user slower than
>>>>>> get_user") and I can tell you it's not trivial given the issue is around
>>>>>> security.  So in practice it does not seem fair to keep a significant
>>>>>> optimization out of kernel because *maybe* we can do it differently even
>>>>>> better :)
>>>>> Maybe a slightly different approach between this patchset and other
>>>>> copy user API would work here. What you want really is something like
>>>>> a temporary mlock on a range of memory so that it is safe for the
>>>>> kernel to access range of userspace virtual address ie page are
>>>>> present and with proper permission hence there can be no page fault
>>>>> while you are accessing thing from kernel context.
>>>>>
>>>>> So you can have like a range structure and mmu notifier. When you
>>>>> lock the range you block mmu notifier to allow your code to work on
>>>>> the userspace VA safely. Once you are done you unlock and let the
>>>>> mmu notifier go on. It is pretty much exactly this patchset except
>>>>> that you remove all the kernel vmap code. A nice thing about that
>>>>> is that you do not need to worry about calling set page dirty it
>>>>> will already be handle by the userspace VA pte. It also use less
>>>>> memory than when you have kernel vmap.
>>>>>
>>>>> This idea might be defeated by security feature where the kernel is
>>>>> running in its own address space without the userspace address
>>>>> space present.
>>>> Like smap?
>>> Yes like smap but also other newer changes, with similar effect, since
>>> the spectre drama.
>>>
>>> Cheers,
>>> Jérôme
>> Sorry do you mean meltdown and kpti?
> Yes all that and similar thing. I do not have the full list in my head.
>
> Cheers,
> Jérôme


Yes, address space of kernel its own is the main motivation of using 
vmap here.

Thanks