[virtio-comment] Re: [PATCH 3/3] content: Document balloon feature free page reporting

[Cornelia Huck <cohuck@redhat.com>]

On Fri, 08 May 2020 10:16:36 -0700
Alexander Duyck <alexander.duyck@gmail.com> wrote:

> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> 
> Free page reporting is a feature that allows the guest to proactively
> report unused pages to the host. By making use of this feature is is
> possible to reduce the overall memory footprint of the guest in cases where
> some significant portion of the memory is idle. Add documentation for the
> free page reporting feature describing the functionality and requirements.
> 
> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> ---
>  content.tex |   75 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 75 insertions(+)
> 
> diff --git a/content.tex b/content.tex
> index e154948a9a1a..97a2c01d498f 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -5007,12 +5007,15 @@ \subsection{Virtqueues}\label{sec:Device Types / Memory Balloon Device / Virtque
>  \item[1] deflateq
>  \item[2] statsq
>  \item[3] free_page_vq
> +\item[4] reporting_vq
>  \end{description}
>  
>    statsq only exists if VIRTIO_BALLOON_F_STATS_VQ is set.
>  
>    free_page_vq only exists if VIRTIO_BALLOON_F_FREE_PAGE_HINT is set.
>  
> +  reporting_vq only exists if VIRTIO_BALLOON_F_PAGE_REPORTING is set.
> +
>  \subsection{Feature bits}\label{sec:Device Types / Memory Balloon Device / Feature bits}
>  \begin{description}
>  \item[VIRTIO_BALLOON_F_MUST_TELL_HOST (0)] Host has to be told before
> @@ -5029,6 +5032,8 @@ \subsection{Feature bits}\label{sec:Device Types / Memory Balloon Device / Featu
>  \item[ VIRTIO_BALLOON_F_PAGE_POISON(4) ] Host has to be notified if guest
>      is expecting reported pages to contain a certain value when returned.
>      Configuration field poison_val is valid.
> +\item[ VIRTIO_BALLOON_F_PAGE_REPORTING(5) ] Device has support for free

s/Device/The device/

> +    page reporting. A virtqueue for reporting free guest memory is present.
>  
>  \end{description}
>  
> @@ -5039,6 +5044,10 @@ \subsection{Feature bits}\label{sec:Device Types / Memory Balloon Device / Featu
>  The driver SHOULD clear the VIRTIO_BALLOON_F_PAGE_POISON flag if it is not
>  expecting any specific value to be stored in the page.
>  
> +The driver MUST NOT accept VIRTIO_BALLOON_F_PAGE_REPORTING if it is expecting
> +the pages to retain some initialized value and it has not negotiated
> +VIRTIO_BALLOON_F_PAGE_POISON as a feature.

"If the driver is expecting the pages to retain some initialized value,
it MUST NOT accept VIRTIO_BALLOON_F_PAGE_REPORTING unless it also
negotiates VIRTIO_BALLOON_F_PAGE_POISON." ?

> +
>  \devicenormative{\subsubsection}{Feature bits}{Device Types / Memory Balloon Device / Feature bits}
>  If the device offers the VIRTIO_BALLOON_F_MUST_TELL_HOST feature
>  bit, and if the driver did not accept this feature bit, the

(...)

> +\drivernormative{\paragraph}{Free Page Reporting}{Device Types / Memory Balloon Device / Device Operation / Free Page Reporting}
> +
> +Normative statements in this section apply if the
> +VIRTIO_BALLOON_F_PAGE_REPORTING feature has been negotiated.
> +
> +If the driver is expecting the free page to contain some initial value it
> +MUST NOT negotiate this feature without negotiating the
> +VIRTIO_BALLOON_F_PAGE_POISON feature as well and supply this value via
> +\field{poison_val}.

Maybe use my suggestion from above (and add the "and supply...")?

> +
> +The driver MUST NOT use the reported pages until the device has
> +acknowledged the reporting request.
> +
> +The driver MAY report free pages any time after DRIVER_OK is set.
> +
> +It is RECOMMENDED that the driver avoid unnecessary reads or writes to the
> +page contents as this could reduce the performance for free page reporting.
> +
> +\devicenormative{\paragraph}{Free Page Reporting}{Device Types / Memory Balloon Device / Device Operation / Free Page Reporting}
> +
> +Normative statements in this section apply if the
> +VIRTIO_BALLOON_F_PAGE_REPORTING feature has been negotiated.
> +
> +The device MAY modify the contents of a page in the report after detecting
> +its physical number in an report request and before acknowledging the
> +reporting request by using the reporting_vq descriptor.

"The device MAY modify the contents of any page supplied in a report
request even before acknowledging that request by using the
reporting_vq descriptor." ?

> +
> +If the VIRTIO_BALLOON_F_PAGE_POISON feature has been negotiated, the device
> +SHALL NOT modify the the page if this will result in the page containing a
> +value other than \field{poison_val}.
> +
>  \section{SCSI Host Device}\label{sec:Device Types / SCSI Host Device}
>  
>  The virtio SCSI host device groups together one or more virtual
> 
> 


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