On 04/04/2022 13:36, Stefan Hajnoczi wrote:
On Wed, Mar 30, 2022 at 04:26:58PM +0100, Usama Arif wrote:@@ -2074,6 +2077,16 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi apply to the queue selected by writing to \field{QueueSel}. } \hline + \mmioreg{DeviceAuxiliaryNotification}{Device Auxiliary Notifier}{0xd0}{W}{ + Writing a value to this register triggers a + notification to the device. The value written to this register is + interpreted as a device auxiliary notification index. The mapping of device auxiliary notifications to + device auxiliary notification indices is device-specific. One possible mapping would be + to use the integers 0 to N-1 as the device auxiliary notification indices for a total of + N device auxiliary notifications. The total number of device auxiliary notifications exposed by the device is + also device-specific. + }Hmm...this is slightly different from the PCI transport's approach. There each dev aux notification has its own hardware register. That has advantages: 1. The notification may carry a value with it because the driver writes a value to the register and the device can interpret that value. That can be used as a performance optimization to reduce the number of hardware register writes performed by the driver or DMA transfers initiated by the device. 2. The dev aux notification registers can be laid out on separate memory pages. This is useful in passthrough scenarios like nested virtualization or slicing up a device in software and passing a subset to an untrusted process. The MMU only offers page-level protection, so MMIO registers that are colocated within the same memory page cannot be selectively passed through to untrusted software. It would be nice if PCI and MMIO exposed dev aux notifications in a similar way. I haven't followed the history of virtio-mmio closely, so I'm not sure if the same approach that was used for PCI is feasible here. Maybe someone else can comment? Stefan
Agree with the points above. I have split the mmio implementation into 2 registers in v2. DeviceAuxNotificationIndex to indicate the device auxiliary notification index and DeviceAuxNotificationData writing to which actually does the notification and carries the data which is device specific.
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