Re: [Xen-devel] [PATCH 3 of 7] docs: add a document describing the xl cfg file syntax

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

Tim, George:

Is this broadly accurate? In particular the bit about why one would use
the shadow_memory option and the suggestion that it also controls the
space used by the HAP overhead.

Cheers,
Ian.

On Mon, 2011-11-07 at 15:13 +0000, Ian Campbell wrote:
> 
> +### Paging
> +
> +The following options control the mechanisms used to virtualise guest
> +memory.  The defaults are selected to give the best results for the
> +common case and so you should normally leave these options
> +unspecified.
> +
> + * `hap=BOOLEAN`: Turns "hardware assisted paging" (the use of the
> +   hardware' nested page table feature) on or off.  Affects HVM
> guests
> +   only.  If turned off, Xen will run the guest in "shadow page
> table"
> +   mode where the guest's page table updates and/or TLB flushes
> +   etc. will be emulated.  Use of HAP is the default when available.
> +
> + * `oos=BOOLEAN`: Turns "out of sync pagetables" on or off.  When
> +   running in shadow page table mode, the guest's page table updates
> +   may be deferred as specified in the Intel/AMD architecture
> manuals.
> +   However this may expose unexpected bugs in the guest, or find bugs
> +   in Xen, so it is possible to disable this feature.  Use of out of
> +   sync page tables, when Xen thinks it appropriate, is the default.
> +
> + * `shadow_memory=MBYTES`: Number of megabytes to set aside for
> +   shadowing guest pagetable pages (effectively acting as a cache of
> +   translated pages) or to use for HAP state. By default this is 1MB
> +   per guest vcpu plus 8KB per MB of guest RAM. You should not
> +   normally need to adjust this value. However if you are not using
> +   hardware assisted paging (i.e. you are using shadow mode) and your
> +   guest workload consists of a large number of processes which do
> not
> +   share address space then increasing this value may improve
> +   performance. 


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