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

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 29 Apr 2020 11:57:57 -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 |   74 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 74 insertions(+)
> 
> diff --git a/content.tex b/content.tex
> index c98b8ea9526a..52955c8ff007 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.

Also lose the trailing period here.

>  \end{description}
>  
>    statsq only exists if VIRTIO_BALLOON_F_STATS_VQ set.
>  
>    free_page_vq only exists if VIRTIO_BALLOON_F_FREE_PAGE_HINT set.
>  
> +  reporting_vq only exists if VIRTIO_BALLOON_F_PAGE_REPORTING set.

s/set/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
> +    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.
> +
>  \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
> @@ -5097,6 +5106,13 @@ \subsection{Device Initialization}\label{sec:Device Types / Memory Balloon Devic
>  
>  \item If the VIRTIO_BALLOON_F_PAGE_POISON feature bit is negotiated then
>    the driver MUST update the poison_val configuration field.
> +
> +\item If the VIRTIO_BALLOON_F_PAGE_REPORTING feature bit is negotiated:
> +  \begin{enumerate}
> +  \item Identify the reporting virtqueue.
> +  \item DRIVER_OK is set: device operation begins.
> +  \item Begin reporting free pages to device.
> +  \end{enumerate}
>  \end{enumerate}

See my previous comments regarding reordering this.

>  
>  \subsection{Device Operation}\label{sec:Device Types / Memory Balloon Device / Device Operation}
> @@ -5497,6 +5513,64 @@ \subsubsection{Page Poison}\label{sec:Device Types / Memory Balloon Device / Dev
>  free page hinting as this feature did not exist prior to those features being
>  added.
>  
> +\subsubsection{Free Page Reporting}\label{sec:Device Types / Memory Balloon Device / Device Operation / Free Page Reporting}
> +
> +Free Page Reporting provides a mechanism similar to balloon inflation,
> +however it does not provide a deflation queue. The expectation is that the
> +device will have a means by which it can detect the guest page access and
> +fault in such pages with some initial value, likely a zero page.
> +
> +The driver will respond to to memory conditions and begin reporting free
> +pages when some number of pages are available.
> +
> +\begin{enumerate}
> +
> +\item The driver determines it has enough pages available to begin
> +  reporting pages.
> +
> +\item The driver gathers pages into a scatter-gather list and adds them to
> +  the reporting_vq.
> +
> +\item The device acknowledges the reporting request.

How?

> +
> +\item Once the device has acknowledged the report, the pages can be
> +  returned to the location from which they were pulled.
> +
> +\item The driver can then continue to gather and report pages until it
> +  has determined it has reported a suffcient quantity of pages.
> +
> +\end{enumerate}
> +
> +\drivernormative{\paragraph}{Free Page Reporting}{Device Types / Memory Balloon Device / Device Operation / Free Page Reporting}
> +
> +Normative statements in this section apply if and only if  the
> +VIRTIO_BALLOON_F_PAGE_REPORTING feature has been negotiated.

"If the VIRTIO_BALLOON_F_PAGE_REPORTING feature has been negotiated:"

> +
> +If the guest is expecting the free page to contain some initial value it

s/guest/driver/

> +MUST make use of the VIRTIO_BALLOON_F_PAGE_POISON feature to notify the
> +device of this expectation via \field{poison_val}.

"..., 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}."

> +
> +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 unecessary 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 and only if  the
> +VIRTIO_BALLOON_F_PAGE_REPORTING feature has been negotiated.

"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

What is a 'physical number'?

> +reporting request by using the reporting_vq descriptor.

What is the 'reporting_vq descriptor'?

> +
> +If the VIRTIO_BALLOON_F_PAGE_POISON feature is negotiated, the device

s/is negotiated/has been negotiated as well/

> +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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