On Tue, 26 Apr 2022 05:52:17 -0400, "Michael S. Tsirkin" <m...@redhat.com> wrote: > On Tue, Apr 26, 2022 at 09:48:29AM +0100, Stefan Hajnoczi wrote: > > On Sun, Apr 24, 2022 at 03:28:53PM +0800, Xuan Zhuo wrote: > > > On Sun, 24 Apr 2022 00:49:19 +0000, Parav Pandit <pa...@nvidia.com> wrote: > > > > Hi, > > > > > > > > A recently defined queue_reset register has a little weird definition > > > > that we should improve. > > > > When driver initiate queue reset, it writes queue_reset = 1. > > > > When device is busy resetting the queue, on this driver request, it is > > > > expected to return queue_reset=0. > > > > Once queue reset is completed it is expected to return queue_reset = 1. > > > > (Polarity changed twice to same value as what was driver set). See more > > > > below. > > > > > > > > So state wise, > > > > # q_enable, q_reset represents : > > > > a) 0,0 -> device init time value > > > > b) 1,0 -> vq is enabled and working > > > > c) 1,1 -> vq is enabled, driver initiated reset > > > > d) 1,0 -> vq is enabled, but device is busy doing the reset > > > > (conflicting definition with above #b ) > > > > e) 0,1 -> vq reset is complete in the device and VQ is now disabled > > > > (again conflict with #a above ) > > > > > > > > Instead, I think we should have below better, consistent definition, no > > > > matter how queue reset occurs (init time or later). > > > > > > > > q_enable, q_reset > > > > A) 0, 0 -> default, device init time > > > > B) 1, 0 -> driver has enabled vq > > > > C) 1, 1 -> driver started q reset > > > > D) 1, 1 -> q_reset stays 1 until device is busy resetting vq > > > > (communicating that its working on resetting, consistent with #C) > > > > E) 0, 0 -> q_reset by device is completed, q got disabled (now matches > > > > the state same as device init time #A) > > > > > > Seems to me to be two different designs, I don't see any particular value > > > in the > > > conflict mentioned here, and under what circumstances would it cause any > > > trouble? > > > > > > The design here is similar to many scenarios, such as device reset. > > > > > > So if you want to change to your design, I want to know if there are other > > > reasons. > > > > A benefit of Parav's suggested definition is that reading the value of > > q_enable and q_reset from the device tells you the state. > > > > The current definition relies on the driver maintaining internal state > > (although this could just be assumptions in the code rather than > > variables stored in memory) because it cannot determine the state by > > reading q_enable and q_reset. > > > > Introspection may not be important for regular drivers because they know > > which previous operations they performed, but in some cases like driver > > crash recovery or live migration introspection can be useful. That said, > > they can reset the entire device to return to a known state, if > > necessary. > > > > Stefan > > Sounds like a valid reason, probably a good idea to include in the > commit log.
In this way, in addition to this reason, one is the complexity of the hardware state. The other is register polarity, I don't understand what special effect this problem has? It seems to be a hardware related problem. Thanks. > > -- > MST > --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscr...@lists.oasis-open.org For additional commands, e-mail: virtio-dev-h...@lists.oasis-open.org