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Re: [Qemu-devel] [RFC 2/2] spec/vhost-user spec: Add IOMMU support

From: Peter Xu
Subject: Re: [Qemu-devel] [RFC 2/2] spec/vhost-user spec: Add IOMMU support
Date: Wed, 12 Apr 2017 15:49:30 +0800
User-agent: Mutt/1.5.24 (2015-08-30)

On Wed, Apr 12, 2017 at 09:24:47AM +0200, Maxime Coquelin wrote:
> On 04/12/2017 09:17 AM, Peter Xu wrote:
> >On Tue, Apr 11, 2017 at 05:16:19PM +0200, Maxime Coquelin wrote:
> >>On 04/11/2017 03:20 PM, Peter Xu wrote:
> >>>On Tue, Apr 11, 2017 at 12:10:02PM +0200, Maxime Coquelin wrote:
> >
> >[...]
> >
> >>>
> >>>>+slave is expected to reply with a zero payload, non-zero otherwise.
> >>>
> >>>Is this ack mechanism really necessary? If not, not sure it'll be nice
> >>>to keep vhost-user/vhost-kernel aligned on this behavior. At least
> >>>that'll simplify vhost-user implementation on QEMU side (iiuc even
> >>>without introducing new functions for update/invalidate operations).
> >>
> >>I think this is necessary, and it won't complexify the vhost-user
> >>implementation on QEMU side, since already widely used (see reply-ack
> >>feature).
> >
> >Could you provide file/function/link pointer to the "reply-ack"
> >feature? I failed to find it myself.
> >
> >>
> >>This reply-ack mechanism is used to obtain a behaviour closer to kernel
> >>backend. Indeed, when QEMU sends a vhost_msg to the kernel backend, it
> >>is blocked in the write() while the message is being processed in the
> >>Kernel. With user backend, QEMU is unblocked from the write() when the
> >>backend has read the message, before it is being processed.
> >>
> >
> >I see. Then I agree with you that we may need a synchronized way to do
> >it. One thing I think of is IOMMU page invalidation - it should be a
> >sync operation to make sure that all the related caches were destroyed
> >when the invalidation command returns in QEMU vIOMMU emulation path.
> >
> >>
> >>>>+
> >>>>+When the VHOST_USER_PROTOCOL_F_SLAVE_REQ is supported by the slave, and 
> >>>>the
> >>>>+master initiated the slave to master communication channel using the
> >>>>+VHOST_USER_SET_SLAVE_REQ_FD request, the slave can send IOTLB miss and 
> >>>>access
> >>>>+failure events by sending VHOST_USER_IOTLB_MSG requests to the master 
> >>>>with a
> >>>>+struct vhost_iotlb_msg payload. For miss events, the iotlb payload has 
> >>>>to be
> >>>>+filled with the miss message type (1), the I/O virtual address and the
> >>>>+permissions flags. For access failure event, the iotlb payload has to be
> >>>>+filled with the access failure message type (4), the I/O virtual address 
> >>>>and
> >>>>+the permissions flags. On success, the master is expected to reply  when 
> >>>>the
> >>>>+request has been handled (for example, on miss requests, once the device 
> >>>>IOTLB
> >>>>+has been updated) with a zero payload, non-zero otherwise.
> >>>
> >>>Failed to understand the last sentence clearly. IIUC vhost-net will
> >>>reply with an UPDATE message when a MISS message is received. Here for
> >>>vhost-user are we going to send one extra zero payload after that?
> >>
> >>Not exactly. There are two channels, one for QEMU to backend requests
> >>(channel A), one for backend to QEMU requests (channel B).
> >>
> >>The backend may be multi-threaded (like DPDK), one thread for handling
> >>QEMU initiated requests (channel A), the others to handle packet
> >>processing (i.e. one for Rx, one for Tx).
> >>
> >>The processing threads will need to translate iova adresses by
> >>searching in the IOTLB cache. In case of miss, it will send an IOTLB
> >>miss request on channel B, and then wait for the ack/nack. In case of
> >>ack, it can search again the IOTLB cache and find the translation.
> >>
> >>On QEMU side, when the thread handling channel B requests receives the
> >>IOTLB miss message, it gets the translation and send an IOTLB update
> >>message on channel A. Then it waits for the ack from the backend,
> >>meaning that the IOTLB cache has been updated, and replies ack on
> >>channel B.
> >
> >If the ack on channel B is used to notify the processing thread that
> >"cache is ready", then... would it be faster that we just let the
> >processing thread poll the cache until it finds it, or let the other
> >thread notify it when it receives ack on channel A? Not sure whether
> >it'll be faster.
> Not sure either.
> Not requiring a ack can indeed make sense in some cases, for example
> with single-threaded backends.
> What we can do is to remove the mandatory ack reply for
> VHOST_USER_IOTLB_MSG slave requests (miss, access fail).
> The backend then can just rely on the REPLY_ACK feature, and set the
> VHOST_USER_NEED_REPLY flag if it want to receive such ack.
> Would it be fine for you?

Okay I found the REPLY_ACK feature now.

It's okay to me (actually, in either way). Thank you.

Peter Xu

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