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

[Wei Liu <wl@xxxxxxx>]

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>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> Release-acked-by: Juergen Gross <jgross@xxxxxxxx>

Acked-by: Wei Liu <wl@xxxxxxx>

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