[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]
Subject: Re: [virtio-comment] Re: [PATCH 07/11] transport-pci: Introduce transitional MMR device id
On Sun, Apr 09, 2023 at 03:15:01AM +0000, Parav Pandit wrote: > > > From: Michael S. Tsirkin <mst@redhat.com> > > Sent: Friday, April 7, 2023 11:51 AM > > > > 1. A non-transitional device will expose a capability (not a feature bit, but a > > capability at transport level). > > > > Note that we can allow this capability in transitional devices too. > > This is useful since IO bar might not be enabled even if present. > > > This capability exposure makes a device transitional in some sense. not in the sense spec uses it at the moment: transitional devices are those that legacy drivers can bind to. transitional drivers btw are those that can bind to legacy devices. perhaps suprisingly, a transitional driver using a transitional device does not rely on any legacy spec at all, they will use the standard interfaces. > > > This capability indicates that, it supports legacy interface. > > > Lets name it legacy_if_emulation for sake of this discussion. > > > It is a two-way pci capability. > > > Device reports it. > > > And driver enables it. (Why two way and why driver needs to enable it, > > described later in point #d below). > > > > > > Hence, such non transitional device does not need to comply to below listed > > requirements #a and #b. > > > > > > a. A driver MUST accept VIRTIO_F_VERSION_1 if it is offered. > > > (Because hypervisor driver is a passthrough driver; and legacy driver will not > > accept this feature bit). > > > > This is not a device requirement at all. > > > Those this is written as driver requirement; a device expects this feature bit to be negotiated. > What should device implementor do? It should allow driver to not negotiate bit, right? > > Which means, below line to be change: > > from: > device MAY fail to operate further if VIRTIO_F_VERSION_1 is not accepted. > > to: > Non transitional device that does not have legacy interface capability MAY fail to operate further if V_1 is not accepted. > Non transitional device that has legacy interface capability SHOULD operate further even if V_1 is not accepted. Look nothing changes with MMR capability at all. We currently have: A device MUST offer VIRTIO_F_VERSION_1. A device MAY fail to operate further if VIRTIO_F_VERSION_1 is not accepted. it's implied that this does not refer to legacy interface. You want to clarify this adding to legacy interface section text explaining that of course VIRTIO_F_VERSION_1 must not be offered through that? Sure but it's a separate issue from MMR capability. don't try to drink the ocean. > > > b. device MAY fail to operate further if VIRTIO_F_VERSION_1 is not accepted. > > > > This is optional not a requirement. > > > Please see above wording, if its acceptable. you don't need any of that for this effort, generally VIRTIO_F_VERSION_1 thing needs a lot of work, if you want to invest the time just ask I'll try to list the issues. But nothing to do with memory mapped legacy interface directly. > > > c. A non-transitional device with above legacy_if_supported > > > capability, will allow device reset sequence, described in [1] Driver > > > Requirements: Device Initialization (3.1.1) [2] Legacy Interface: > > > Device Initialization (3.1.2) > > > > > > > > device reset sequence. > > > > > > > > what is this one? > > > > > > I listed above in #c. > > > And > > > > > > d. When legacy_if_emulation capability is offered and hypervisor driver > > enabled it, when driver perform device reset, driver will not wait for device > > reset to go zero. > > > When legacy_if_emulation capability is not enabled by (hypervisor or other > > say existing) driver, driver will wait for device reset to turn 0. (Following the > > driver requirement 2.4.2). > > > > It might not be a bad idea to enable it, but I observe that it is possible for > > hypervisor to expose a standard transitional device on top of this MMR > > capability. Thus it will not be known whether guest driver accesses legacy or > > modern BAR until guest runs. > > I propose, instead, that device exposes same registers at two addresses and > > executes reset correctly depending on which address it was accessed through. > > WDYT? > Yep, this the exact proposal here. > Legacy registers exposes via AQ (aka TVQ) or MMR location, behaves like legacy. > And regular registers at their location as-is. > > With that feature bit negotiation is the only thing to relax like worded above. It's not really different from IO port legacy then. -- MST
[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]