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Subject: Re: [virtio-comment] [PATCH 2/3] content: Document balloon feature page poison
On 08.05.20 19:16, Alexander Duyck wrote: > From: Alexander Duyck <alexander.h.duyck@linux.intel.com> > > Page poison provides a way for the guest to notify the host of the content > expected to be found in pages when they are added back to the guest after > being discarded. The feature currently doesn't apply to the existing > balloon features, however it will apply to an upcoming feature, free page > reporting. Add documentation for the page poison feature describing the > basic functionality and requirements. > > Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com> > --- > content.tex | 45 +++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 45 insertions(+) > > diff --git a/content.tex b/content.tex > index 7d91604178fd..e154948a9a1a 100644 > --- a/content.tex > +++ b/content.tex > @@ -5026,6 +5026,9 @@ \subsection{Feature bits}\label{sec:Device Types / Memory Balloon Device / Featu > page hinting. A virtqueue for providing hints as to what memory is > currently free is present. Configuration field free_page_hint_cmd_id > is valid. > +\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. > > \end{description} > > @@ -5033,6 +5036,9 @@ \subsection{Feature bits}\label{sec:Device Types / Memory Balloon Device / Featu > The driver SHOULD accept the VIRTIO_BALLOON_F_MUST_TELL_HOST > feature if offered by the device. > > +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. > + > \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 > @@ -5055,11 +5061,15 @@ \subsection{Device configuration layout}\label{sec:Device Types / Memory Balloon > VIRTIO_BALLOON_F_FREE_PAGE_HINT has been negotiated and is read-only by > the driver. > > + \field{poison_val} is available if VIRTIO_BALLOON_F_PAGE_POISON has been > + negotiated. > + > \begin{lstlisting} > struct virtio_balloon_config { > le32 num_pages; > le32 actual; > le32 free_page_hint_cmd_id; > + le32 poison_val; > }; > \end{lstlisting} > > @@ -5088,6 +5098,9 @@ \subsection{Device Initialization}\label{sec:Device Types / Memory Balloon Devic > \item If the VIRTIO_BALLOON_F_FREE_PAGE_HINT feature bit is negotiated the > free_page_vq is identified. > > +\item If the VIRTIO_BALLOON_F_PAGE_POISON feature bit is negotiated then > + the driver MUST update the poison_val configuration field. > + > \item DRIVER_OK is set: device operation begins. > > \item If the VIRTIO_BALLOON_F_STATS_VQ feature bit is negotiated then > @@ -5461,6 +5474,38 @@ \subsubsection{Free Page Hinting}\label{sec:Device Types / Memory Balloon Device > The device MAY NOT modify the contents of the balloon after > \field{free_page_hint_cmd_id} is set to VIRTIO_BALLOON_CMD_ID_DONE. > > +\subsubsection{Page Poison}\label{sec:Device Types / Memory Balloon Device / Device Operation / Page Poison} > + > +Page Poison provides a way to notify the host of the contents that are > +currently in the balloon pages, and those that are expected to be in the > +pages when they are pulled from the balloon. It is used for in-place "when they are pulled from the balloon". That's not correct. This only applies to free page reporting (-> patch #3). Without free page reporting, poisoning only tells the hypervisor that pages pages that are getting deflated might immediately be written by the hypervisor again. Or am I missing something? > +reporting of pages without needing to pull them from the memory allocator > +of the guest. > + > +If VIRTIO_BALLOON_F_PAGE_POISON feature has been negotiated, the guest > +will place the expected poison value in \field{poison_val} configuration > +data. > + > +\drivernormative{\paragraph}{Page Poison}{Device Types / Memory Balloon Device / Device Operation / Page Poison} > + > +Normative statements in this section apply if the > +VIRTIO_BALLOON_F_PAGE_POISON feature has been negotiated. > + > +The driver MUST populate the \field{poison_val} configuration data if it is > +expecting the page to contain some fixed value when free. Again, not correctly phrased I think. Only applies to free page reporting. > + > +The driver MAY opt to disable the feature if it will take care of > +re-initializing pages when first accessing them. > + > +\devicenormative{\paragraph}{Page Poison}{Device Types / Memory Balloon Device / Device Operation / Page Poison} > + > +Normative statements in this section apply if the > +VIRTIO_BALLOON_F_PAGE_POISON feature has been negotiated. > + > +The device MAY ignore the \field{poison_val} for normal balloon operations and > +free page hinting as this feature did not exist prior to those features being > +added. The device can always ignore this field without free page reporting. It's only a hint that the guest is using poisoning, nothing else. -- Thanks, David / dhildenb
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