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Subject: Re: [virtio-comment] Re: virtio-sound: release control request clarification
Hi Matias, On 18.10.2023 22:30, Matias Ezequiel Vara Larsen wrote:
Hello Anton, thanks for the response. I added some inline comments. On Wed, Oct 18, 2023 at 10:06:05AM +0900, Anton Yakovlev wrote:Hi Matias, On 18.10.2023 00:19, Matias Ezequiel Vara Larsen wrote:Hello, This email is to clarify the VirtIO specification regarding the RELEASE control request. Section 5.14.6.6.5.1 [1] states the following device requirements for the RELEASE control request: 1. The device MUST complete all pending I/O messages for the specified stream ID. 2. The device MUST NOT complete the control request while there are pending I/O messages for the specified stream ID. The 1) requirement does not indicate what "complete" means. Does it mean that the pending I/O messages in the tx queue shall be outputted in the host, i.e., consumed by the audio backend? Or, completion means simply to put the requests in the used-ring without consuming them?Here "to complete" means moving the buffers to the used list in vring. Technically, the specification only requires that the device "return" all referenced DMA memory to the guest before completing the RELEASE control request. What the device actually does with these I/O messages is implementation dependent and is not within the scope of the specification. Thus...Thank you, I got it. If I correctly understand you, after RELEASE is issued, the specs specify only that the device should "return" all buffers or "complete" them. Device implementations MAY or MAY NOT playback them. In other words, the specification does not specify if consumption should occur. I had interpreted this to mean that the guest intended to output those buffers, leaving the device implementation with no option but to do so.Regarding 2), I interpret it as "the device shall wait until all I/O messages are proceeded to complete the RELEASE control request"....you can do this way if you really need to.Currently, the kernel driver seems not expecting such a delay when the RELEASE command is sent. If I understand correctly, the kernel driver first sends the RELEASE command and waits a fixed amount of time until the device can process it. Then, the driver waits a fixed amount of time until all pending IO messages are completed. If the device follows the specification and waits until all messages IO are completed to issue the completion of the RELEASE command, the kernel driver may timeout. The time to complete N IO messages in the TX queue could be proportional with the number of pending messages.The default timeout for control requests in the ALSA driver is 1 second. In theory, this time should be enough to completely reproduce/fill the 500ms buffer, and complete all requests, including the RELEASE control request. If the device fails to do this, then most likely there are some problems with the implementation.Thanks for clarifying. Sorry to repeat myself, the point I want to make is that the virtsnd_pcm_sync_stop() function that sends the RELEASE control request uses virtsnd_ctl_msg_send_sync(). Message timeouts are set up by the "msg_timeout_ms" module parameter. The timeout is the same as for other control requests, such as SET_PARAM and PREPARE, but these commands do not require flushing a queue, so I wondered how the timeout could be the same.
In general, I don't think it's possible to scale/adjust the timeout value for any of the control requests at runtime. You pointed only at the RELEASE request, but depending on the device implementation, other requests may also take an unpredictable amount of time. For example, if the device communicates with an audio server over the network. Therefore, in order not to overcomplicate things, a single more or less reasonable default value was chosen. Which is also suitable for RELEASE (see my thoughts above). Best regards,
In our device implementation [2], RELEASE is handled as follows: - Drop all messages in the TX queue without outputting in the host. - Complete the RELEASE control request. This seems to be working, however, I can observe that sometimes there are still requests in the TX queue when we get RELEASE. Those requests are never reproduced in the host. My questions are: - In the specification, should we modify it to clarify that all pending IO messages in the device are discarded during RELEASE, that is, not output to the host, but signaled to the guest as completed?No, we shouldn't. See comment above.- According to the specification, should the driver wait in RELEASE an amount of time proportional to the number of periods yet to be reproduced?This is purely a matter of driver implementation. It is possible to implement the driver without timeouts, but this would be a bad idea. Because bugs in the device could lead to an infinite wait in the kernel.I agree, thanks. Matias.Best regards,Thanks, Matias. [1] https://ddec1-0-en-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2fdocs.oasis%2dopen.org%2fvirtio%2fvirtio%2fv1.2%2fcsd01%2fvirtio%2dv1.2%2dcsd01.html&umid=d5297ffc-9da6-41eb-a09f-b57cd7282232&auth=53c7c7de28b92dfd96e93d9dd61a23e634d2fbec-411b9d1d7b38e7727e6478284b6313d5ad82f5a5 [2] https://github.com/rust-vmm/vhost-device/tree/main/staging/vhost-device-sound-- Anton Yakovlev Senior Software Engineer OpenSynergy GmbH Rotherstr. 20, 10245 Berlin This publicly archived list offers a means to provide input to the OASIS Virtual I/O Device (VIRTIO) TC. In order to verify user consent to the Feedback License terms and to minimize spam in the list archive, subscription is required before posting. Subscribe: virtio-comment-subscribe@lists.oasis-open.org Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org List help: virtio-comment-help@lists.oasis-open.org List archive: https://ddec1-0-en-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2flists.oasis%2dopen.org%2farchives%2fvirtio%2dcomment%2f&umid=d5297ffc-9da6-41eb-a09f-b57cd7282232&auth=53c7c7de28b92dfd96e93d9dd61a23e634d2fbec-a481d373f80a6c1b5882e1f0170a0687329391b1 Feedback License: https://ddec1-0-en-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2fwww.oasis%2dopen.org%2fwho%2fipr%2ffeedback%5flicense.pdf&umid=d5297ffc-9da6-41eb-a09f-b57cd7282232&auth=53c7c7de28b92dfd96e93d9dd61a23e634d2fbec-9797dad096ce101e2ddeccc4601649f954e89c09 List Guidelines: https://ddec1-0-en-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2fwww.oasis%2dopen.org%2fpolicies%2dguidelines%2fmailing%2dlists&umid=d5297ffc-9da6-41eb-a09f-b57cd7282232&auth=53c7c7de28b92dfd96e93d9dd61a23e634d2fbec-e529fd6878ce2da5bc6d951687d9f90e34808b1b Committee: https://ddec1-0-en-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2fwww.oasis%2dopen.org%2fcommittees%2fvirtio%2f&umid=d5297ffc-9da6-41eb-a09f-b57cd7282232&auth=53c7c7de28b92dfd96e93d9dd61a23e634d2fbec-afc2604472b6094aee00bdd9a5aa0a65d12f7e19 Join OASIS: https://ddec1-0-en-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2fwww.oasis%2dopen.org%2fjoin%2f&umid=d5297ffc-9da6-41eb-a09f-b57cd7282232&auth=53c7c7de28b92dfd96e93d9dd61a23e634d2fbec-cae044bc06deaf94443835e5f72f10909d5abeec
-- Anton Yakovlev Senior Software Engineer OpenSynergy GmbH Rotherstr. 20, 10245 Berlin
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