Re: [Qemu-devel] [PATCH 3/6] vhost: Update rings information for IOTLB earlier

[Michael S. Tsirkin]

On Thu, May 18, 2017 at 04:45:23PM +0200, Maxime Coquelin wrote:
> Hi Michael,
> 
> On 05/18/2017 09:35 AM, Maxime Coquelin wrote:
> > 
> > 
> > On 05/17/2017 06:41 PM, Michael S. Tsirkin wrote:
> > > On Fri, May 12, 2017 at 01:21:18PM +0200, Maxime Coquelin wrote:
> > > > 
> > > > On 05/11/2017 07:33 PM, Michael S. Tsirkin wrote:
> > > > > On Thu, May 11, 2017 at 02:32:43PM +0200, Maxime Coquelin wrote:
> > > > > > Vhost-kernel backend need to receive IOTLB entries for rings
> > > > > > information early, but vhost-user need the same information
> > > > > > earlier, before VHOST_USER_SET_VRING_ADDR is sent.
> > > > > Weird. What does VHOST_USER_SET_VRING_ADDR have to do with it?
> > > > > 
> > > > > According to
> > > > >     Starting and stopping rings
> > > > > in vhost user spec, vhost user does not access
> > > > > anything until ring is started and enabled.
> > > > > 
> > > > > 
> > > > > > This patch also trigger IOTLB miss for all rings informations
> > > > > > for robustness, even if in practice these adresses are on the
> > > > > > same page.
> > > > Actually, the DPDK vhost-user backend is compliant with the spec,
> > > > but when handling VHOST_USER_SET_VRING_ADDR request, it translates the
> > > > guest addresses into backend VAs, and check they are valid. I
> > > > will make the
> > > > commit message clearer about this in next revision.
> > > > 
> > > > The check could be done later, for example when the ring are started,
> > > > but it wouldn't change the need to trigger a miss at some point.
> > > I think it should be done later, yes. As long as ring is not
> > > started addresses should not be interpreted.
> > > 
> > 
> > Ok, then I'll move these addresses translations in the
> > VHOST_USER_SET_VRING_KICK handler.
> s/VHOST_USER_SET_VRING_KICK/VHOST_USER_SET_VRING_ENABLE/

Note that when protocol features are off ring is started in
enabled state, but iommu requires protocol features.

> I just looked at implementing this change, but I'm not convinced this is
> the right thing to do.
> 
> On backend side, it means saving temporarily the vhost_vring_addr struct
> into the vq struct, and moving all what is done currently in
> SET_VRING_ADDR handler to SET_VRING_ENABLE one.

Yes, and this is consistent with what the kernel does.

> My understanding of the "Starting and stopping rings" chapter of the
> spec is that the ring must not be processed as long as not started and
> enabled, not that the addresses passed should not be checked/translated
> as it is done today both in DPDK and libvhost-user.
> 
> If the addresses are invalid, isn't it better to know as soon as
> possible?
> 
> Cheers,
> Maxime

There could be valid reasons to set an invalid address temporarily.
For example to make sure connection is reset.



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