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Subject: Re: [PATCH RFC 3/3] content: Document balloon feature free page reporting
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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