Re: [PATCH v2 1/1] transport-mmio: Add v3, which polls for reset completion

[Matias Ezequiel Vara Larsen <mvaralar@redhat.com>]

On Thu, Apr 16, 2026 at 06:52:56PM +0200, Peter Hilber wrote:
> Let devices using the MMIO transport avoid stalling the driver (virtual)
> CPU during device reset, which requires introducing a new MMIO transport
> version.
> 
> Unlike the PCI transport, the MMIO transport does not require the driver
> to poll for reset completion. This requires a device using the MMIO
> transport to complete reset during the write of zero to the Status
> register. Device reset may take more than 100 ms if it involves
> terminating ongoing device activity which accesses driver memory. When
> the (virtual) CPU writing zero to the Status register needs to be
> stalled during this, this may violate real-time requirements (including
> those for hypervisor trap-and-emulate).
> 
> Address this by introducing a new MMIO transport version, v3, where the
> driver must poll for reset completion, and, hence, the device reset does
> not have to complete during the write to the Status register. Add a
> requirement that resetting a device which already has Status zero is
> synchronous to avoid ambiguity when such a reset would finish.
> 
> For clarity, also add some related requirements for v2. These
> requirements are implied by the rest of the specification and therefore
> do not alter the v2 semantics.
> 
> With MMIO transport v3, reset essentially works as with the PCI
> transport, and the change is therefore not expected to cause problems.
> 
> Existing devices with MMIO transport v2 are not required to implement
> v3, which will not work with current drivers. Only devices which require
> asynchronous reset need to report MMIO transport v3, and therefore
> require an updated driver.
> 
> Drivers have to support the MMIO transport versions of the used devices.
> Portable MMIO transport drivers should therefore support both v2 and v3,
> which is simple.
> 
> Signed-off-by: Peter Hilber <peter.hilber@oss.qualcomm.com>
> ---


Reviewed-by: Matias Ezequiel Vara Larsen <mvaralar@redhat.com>

Matias

> 
> Notes:
>     v2:
>     
>     - Add summary of MMIO v3 to Version register description (Matias)
>     
>     - Align requirements on Status register transition to zero with transport-
>       independent requirements (Matias)
>     
>     - Change requirements relating to reset when status is already zero
>       (Matias)
>     
>     - In the commit message, document when v3 devices may be incompatible with
>       v2 drivers (as per reply to Michael)
>     
>     - Improve wording; refer to "zero" rather than "0"
> 
>  transport-mmio.tex | 33 ++++++++++++++++++++++++++++++---
>  1 file changed, 30 insertions(+), 3 deletions(-)
> 
> diff --git a/transport-mmio.tex b/transport-mmio.tex
> index 94a93a1..cddbcc9 100644
> --- a/transport-mmio.tex
> +++ b/transport-mmio.tex
> @@ -64,7 +64,8 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi
>    }
>    \hline
>    \mmioreg{Version}{Device version number}{0x004}{R}{%
> -    0x2.
> +    0x2 or 0x3. With version 0x3, the driver waits until it reads zero from the
> +    \field{Status} register before considering a reset complete.
>      \begin{note}
>        Legacy devices (see \ref{sec:Virtio Transport Options / Virtio Over MMIO / Legacy interface}~\nameref{sec:Virtio Transport Options / Virtio Over MMIO / Legacy interface}) used 0x1.
>      \end{note}
> @@ -185,6 +186,11 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi
>      Writing non-zero values to this register sets the status flags,
>      indicating the driver progress. Writing zero (0x0) to this
>      register triggers a device reset.
> +
> +    Starting with \field{Version} 0x3, the device reset need not
> +    complete before the write returns, and the driver waits until it
> +    reads back zero before considering a reset complete.
> +
>      See also p. \ref{sec:Virtio Transport Options / Virtio Over MMIO / MMIO-specific Initialization And Device Operation / Device Initialization}~\nameref{sec:Virtio Transport Options / Virtio Over MMIO / MMIO-specific Initialization And Device Operation / Device Initialization}.
>    }
>    \hline
> @@ -262,13 +268,31 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi
>  
>  The device MUST return 0x74726976 in \field{MagicValue}.
>  
> -The device MUST return value 0x2 in \field{Version}.
> +The device MUST return value 0x2 or 0x3 in \field{Version}.
>  
>  The device MUST present each event by setting the corresponding bit in \field{InterruptStatus} from the
>  moment it takes place, until the driver acknowledges the interrupt
>  by writing a corresponding bit mask to the \field{InterruptACK} register.  Bits which
>  do not represent events which took place MUST be zero.
>  
> +The device MUST reset when zero is written to \field{Status}.
> +
> +The device MUST present zero in \field{Status} once it has finished the reset.
> +
> +For \field{Version} 0x2, the device MUST finish a reset before the driver's
> +write of zero to \field{Status} has completed.
> +
> +For \field{Version} 0x3, if \field{Status} is non-zero before a reset, the
> +device MAY continue with the reset after the driver's write of zero to
> +\field{Status} has completed.
> +
> +For \field{Version} 0x3, if \field{Status} is zero before a reset, the device
> +MUST finish the reset before the driver's write of zero to \field{Status} has
> +completed.
> +
> +For \field{Version} 0x3, the device MUST change \field{Status} to zero as the
> +last step of the reset.
> +
>  Upon reset, the device MUST clear all bits in \field{InterruptStatus} and ready bits in the
>  \field{QueueReady} register for all queues in the device.
>  
> @@ -305,7 +329,7 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi
>  The driver MUST ignore a device with \field{MagicValue} which is not 0x74726976,
>  although it MAY report an error.
>  
> -The driver MUST ignore a device with \field{Version} which is not 0x2,
> +The driver MUST ignore a device whose \field{Version} is neither 0x2 nor 0x3,
>  although it MAY report an error.
>  
>  The driver MUST ignore a device with \field{DeviceID} 0x0,
> @@ -331,6 +355,9 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi
>  The driver MUST write a value with a bit mask describing events it handled into \field{InterruptACK} when
>  it finishes handling an interrupt and MUST NOT set any of the undefined bits in the value.
>  
> +For \field{Version} 0x3, the driver MUST NOT consider a reset complete before
> +reading back zero in \field{Status}.
> +
>  If VIRTIO_F_RING_RESET has been negotiated, after the driver writes 1 to
>  \field{QueueReset} to reset the queue, the driver MUST NOT consider queue
>  reset to be complete until it reads back 0 in \field{QueueReset}. The driver
> -- 
> 2.43.0
>