Re: [virtio] [PATCH 3/3] VIRTIO-110: ARM's feedback for MMIO chapter, clarifications

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

On Mon, Aug 04, 2014 at 07:08:01PM +0100, Pawel Moll wrote:
> Those changes do not add nor remove any features and constitutes
> only error correction and editorial changes.

This one seems to be borderline.

> * Extra clarifications for QueueReady and ConfigGeneration
> * Re-added alignment requirement section - it existed in the
>   original version of the spec, so no extra limitations here.

What is an "original version"? I could not find this in draft 01.

> Signed-off-by: Pawel Moll <pawel.moll@arm.com>
> ---
>  content.tex | 21 ++++++++++++++++-----
>  1 file changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/content.tex b/content.tex
> index 17ba3b8..769bef2 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -1997,8 +1997,8 @@ All register values are organized as Little Endian.
>    }
>    \hline 
>    \mmioreg{QueueReady}{Virtual queue ready bit}{0x044}{RW}{%
> -    Writing one (0x1) to this register notifies the device that the
> -    virtual queue is ready to be used. Reading from this register
> +    Writing one (0x1) to this register notifies the device that it can
> +    execute requests from this virtual queue. Reading from this register
>      returns the last value written to it. Both read and write
>      accesses apply to the queue selected by writing to \field{QueueSel}.
>    }
> @@ -2058,7 +2058,11 @@ All register values are organized as Little Endian.
>    }
>    \hline 
>    \mmioreg{ConfigGeneration}{Configuration atomicity value}{0x0fc}{R}{
> -    Changes every time the configuration noticeably changes (see \ref {sec:Basic Facilities of a Virtio Device / Device Configuration Space}.
> +    Reading from this register returns a value describing a version of the device-specific configuration (see \field{Config}).
> +    The driver can then access the configuration space and then read \field{ConfigGeneration} again.
> +    If no part of the configuration space has changed, \field{ConfigGeneration} returns the same value.
> +    If the value is different, the configuration space access was not atomic and the driver must perform the operation again.
> +    See also \ref {sec:Basic Facilities of a Virtio Device / Device Configuration Space}.
>    }
>    \hline 
>    \mmioreg{Config}{Configuration space}{0x100+}{RW}{
> @@ -2083,10 +2087,12 @@ do not represent events which took place MUST be zero.
>  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.
>  
> -The device MUST change \field{ConfigGeneration} if there is any risk of a
> -device seeing an inconsistent configuration state, but it MAY only change the value
> +The device MUST change value returned in \field{ConfigGeneration} if there is any risk of a
> +driver seeing an inconsistent configuration state, but it MAY only change the value
>  after a configuration read operation.

"MAY only" is really confusing. You are using MAY here to
describe a mandatory requirement. Can be re-written with "MUST NOT"?
Also, surely reset changes this field to 0?


>  
> +The device MUST NOT access virtual queue contents when \field{QueueReady} is zero (0x0).
> +
>  \drivernormative{\subsubsection}{MMIO Device Register Layout}{Virtio Transport Options / Virtio Over MMIO / MMIO Device Register Layout}
>  The driver MUST NOT access memory locations not described in the
>  table \ref{tab:Virtio Trasport Options / Virtio Over MMIO / MMIO Device Register Layout}
> @@ -2094,6 +2100,11 @@ table \ref{tab:Virtio Trasport Options / Virtio Over MMIO / MMIO Device Register
>  MUST NOT write to the read-only registers (direction R) and
>  MUST NOT read from the write-only registers (direction W).
>  
> +The driver MUST only use 32 bit wide and aligned reads and writes to access the control registers
> +described in table \ref{tab:Virtio Trasport Options / Virtio Over MMIO / MMIO Device Register Layout}.

OK this was kind of implied all the time ... you think adding this text
is error correction?

> +The driver MAY use other accesses for the device-specific configuration space,
> +depending on the device type.
> +

Why are you adding this option?
If there's a 4 byte register it is a pain for devices to support
1 byte accesses, and we gain nothing from it.


>  The driver MUST ignore a device with \field{MagicValue} which is not 0x74726976,
>  although it MAY report an error.
>  
> -- 
> 1.9.1
> 
> 
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