Re: [Xen-devel] [PATCH-for-4.13 v7] Rationalize max_grant_frames and max_maptrack_frames handling

[George Dunlap <George.Dunlap@xxxxxxxxxx>]

> On Nov 29, 2019, at 5:44 PM, Wei Liu <wl@xxxxxxx> wrote:
> 
> On Fri, Nov 29, 2019 at 05:36:11PM +0000, Wei Liu wrote:
>> On Fri, Nov 29, 2019 at 05:24:45PM +0000, Paul Durrant wrote:
>>> From: George Dunlap <george.dunlap@xxxxxxxxxx>
>>> 
>>> Xen used to have single, system-wide limits for the number of grant
>>> frames and maptrack frames a guest was allowed to create. Increasing
>>> or decreasing this single limit on the Xen command-line would change
>>> the limit for all guests on the system.
>>> 
>>> Later, per-domain limits for these values was created. The system-wide
>>> limits became strict limits: domains could not be created with higher
>>> limits, but could be created with lower limits. However, that change
>>> also introduced a range of different "default" values into various
>>> places in the toolstack:
>>> 
>>> - The python libxc bindings hard-coded these values to 32 and 1024,
>>>  respectively
>>> - The libxl default values are 32 and 1024 respectively.
>>> - xl will use the libxl default for maptrack, but does its own default
>>>  calculation for grant frames: either 32 or 64, based on the max
>>>  possible mfn.
>>> 
>>> These defaults interact poorly with the hypervisor command-line limit:
>>> 
>>> - The hypervisor command-line limit cannot be used to raise the limit
>>>  for all guests anymore, as the default in the toolstack will
>>>  effectively override this.
>>> - If you use the hypervisor command-line limit to *reduce* the limit,
>>>  then the "default" values generated by the toolstack are too high,
>>>  and all guest creations will fail.
>>> 
>>> In other words, the toolstack defaults require any change to be
>>> effected by having the admin explicitly specify a new value in every
>>> guest.
>>> 
>>> In order to address this, have grant_table_init treat negative values
>>> for max_grant_frames and max_maptrack_frames as instructions to use the
>>> system-wide default, and have all the above toolstacks default to passing
>>> -1 unless a different value is explicitly configured.
>>> 
>>> This restores the old behavior in that changing the hypervisor command-line
>>> option can change the behavior for all guests, while retaining the ability
>>> to set per-guest values.  It also removes the bug that reducing the
>>> system-wide max will cause all domains without explicit limits to fail.
>>> 
>>> NOTE: - The Ocaml bindings require the caller to always specify a value,
>>>        and the code to start a xenstored stubdomain hard-codes these to 4
>>>     and 128 respectively; this behavour will not be modified.
>>> 
>>> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
>>> Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
>>> Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>

Does this win some kind of award for “most collaborative patch”?

 -George

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