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From: "Liang, Cunming" <email@example.com>
To: "Bie, Tiwei" <firstname.lastname@example.org>,
"Michael S. Tsirkin" <email@example.com>
Cc: Jason Wang <firstname.lastname@example.org>,
"Duyck, Alexander H" <email@example.com>,
"Daly, Dan" <firstname.lastname@example.org>,
"Wang, Zhihong" <email@example.com>,
"Tan, Jianfeng" <firstname.lastname@example.org>,
"Wang, Xiao W" <email@example.com>,
"Tian, Kevin" <firstname.lastname@example.org>
Subject: RE: [RFC] vhost: introduce mdev based hardware vhost backend
Date: Fri, 20 Apr 2018 03:50:41 +0000 [thread overview]
Message-ID: <D0158A423229094DA7ABF71CF2FA0DA34E9511D5@SHSMSX104.ccr.corp.intel.com> (raw)
> -----Original Message-----
> From: Bie, Tiwei
> Sent: Friday, April 20, 2018 11:28 AM
> To: Michael S. Tsirkin <email@example.com>
> Cc: Jason Wang <firstname.lastname@example.org>; email@example.com;
> firstname.lastname@example.org; Duyck, Alexander H <email@example.com>;
> firstname.lastname@example.org; email@example.com;
> firstname.lastname@example.org; email@example.com;
> firstname.lastname@example.org; Daly, Dan <email@example.com>; Liang, Cunming
> <firstname.lastname@example.org>; Wang, Zhihong <email@example.com>; Tan,
> Jianfeng <firstname.lastname@example.org>; Wang, Xiao W <email@example.com>;
> Tian, Kevin <firstname.lastname@example.org>
> Subject: Re: [RFC] vhost: introduce mdev based hardware vhost backend
> On Thu, Apr 19, 2018 at 09:40:23PM +0300, Michael S. Tsirkin wrote:
> > On Tue, Apr 10, 2018 at 03:25:45PM +0800, Jason Wang wrote:
> > > > > > One problem is that, different virtio ring compatible devices
> > > > > > may have different device interfaces. That is to say, we will
> > > > > > need different drivers in QEMU. It could be troublesome. And
> > > > > > that's what this patch trying to fix. The idea behind this
> > > > > > patch is very simple: mdev is a standard way to emulate device
> > > > > > in kernel.
> > > > > So you just move the abstraction layer from qemu to kernel, and
> > > > > you still need different drivers in kernel for different device
> > > > > interfaces of accelerators. This looks even more complex than
> > > > > leaving it in qemu. As you said, another idea is to implement
> > > > > userspace vhost backend for accelerators which seems easier and
> > > > > could co-work with other parts of qemu without inventing new type of
> > > > I'm not quite sure. Do you think it's acceptable to add various
> > > > vendor specific hardware drivers in QEMU?
> > > >
> > >
> > > I don't object but we need to figure out the advantages of doing it
> > > in qemu too.
> > >
> > > Thanks
> > To be frank kernel is exactly where device drivers belong. DPDK did
> > move them to userspace but that's merely a requirement for data path.
> > *If* you can have them in kernel that is best:
> > - update kernel and there's no need to rebuild userspace
> > - apps can be written in any language no need to maintain multiple
> > libraries or add wrappers
> > - security concerns are much smaller (ok people are trying to
> > raise the bar with IOMMUs and such, but it's already pretty
> > good even without)
> > The biggest issue is that you let userspace poke at the device which
> > is also allowed by the IOMMU to poke at kernel memory (needed for
> > kernel driver to work).
> I think the device won't and shouldn't be allowed to poke at kernel memory. Its
> kernel driver needs some kernel memory to work. But the device doesn't have
> the access to them. Instead, the device only has the access to:
> (1) the entire memory of the VM (if vIOMMU isn't used) or
> (2) the memory belongs to the guest virtio device (if
> vIOMMU is being used).
> Below is the reason:
> For the first case, we should program the IOMMU for the hardware device based
> on the info in the memory table which is the entire memory of the VM.
> For the second case, we should program the IOMMU for the hardware device
> based on the info in the shadow page table of the vIOMMU.
> So the memory can be accessed by the device is limited, it should be safe
> especially for the second case.
> My concern is that, in this RFC, we don't program the IOMMU for the mdev
> device in the userspace via the VFIO API directly. Instead, we pass the memory
> table to the kernel driver via the mdev device (BAR0) and ask the driver to do the
> IOMMU programming. Someone may don't like it. The main reason why we don't
> program IOMMU via VFIO API in userspace directly is that, currently IOMMU
> drivers don't support mdev bus.
> > Yes, maybe if device is not buggy it's all fine, but it's better if we
> > do not have to trust the device otherwise the security picture becomes
> > more murky.
> > I suggested attaching a PASID to (some) queues - see my old post
> > "using PASIDs to enable a safe variant of direct ring access".
Ideally we can have a device binding with normal driver in host, meanwhile support to allocate a few queues attaching with PASID on-demand. By vhost mdev transport channel, the data path ability of queues(as a device) can expose to qemu vhost adaptor as a vDPA instance. Then we can avoid VF number limitation, providing vhost data path acceleration in a small granularity.
> It's pretty cool. We also have some similar ideas.
> Cunming will talk more about this.
> Best regards,
> Tiwei Bie
> > Then using IOMMU with VFIO to limit access through queue to corrent
> > ranges of memory.
> > --
> > MST
next prev parent reply other threads:[~2018-04-20 3:50 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-02 15:23 Tiwei Bie
2018-04-10 2:52 ` Jason Wang
2018-04-10 4:57 ` Tiwei Bie
2018-04-10 7:25 ` Jason Wang
2018-04-19 18:40 ` Michael S. Tsirkin
2018-04-20 3:28 ` Tiwei Bie
2018-04-20 3:50 ` Michael S. Tsirkin
2018-04-20 3:50 ` Liang, Cunming [this message]
2018-04-20 13:52 ` Michael S. Tsirkin
2018-04-20 3:52 ` Jason Wang
2018-04-20 14:12 ` Michael S. Tsirkin
2018-04-10 7:51 ` [virtio-dev] " Paolo Bonzini
2018-04-10 9:23 ` Liang, Cunming
2018-04-10 13:36 ` Michael S. Tsirkin
2018-04-10 14:23 ` Liang, Cunming
2018-04-11 1:38 ` Tian, Kevin
2018-04-11 2:18 ` Jason Wang
2018-04-11 2:01 ` Stefan Hajnoczi
2018-04-11 2:08 ` Jason Wang
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