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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Feb 04, 2026 at 12:50:22PM +0100, 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 0 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 0 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.
> 
> 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. 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>
> ---
>  transport-mmio.tex | 26 +++++++++++++++++++++++---
>  1 file changed, 23 insertions(+), 3 deletions(-)
> 
> diff --git a/transport-mmio.tex b/transport-mmio.tex
> index 94a93a1..6504a6b 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 0 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}

But this means there's no way to support existing (non polling) and new (polling)
drivers both in the same hypervisor:

        /* Check device version */
        vm_dev->version = readl(vm_dev->base + VIRTIO_MMIO_VERSION);
        if (vm_dev->version < 1 || vm_dev->version > 2) {
                dev_err(&pdev->dev, "Version %ld not supported!\n",
                                vm_dev->version);
                rc = -ENXIO;
                goto free_vm_dev;
        }



Don't we want to?



-- 
MST