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

[Tim Deegan <tim@xxxxxxx>]

At 08:10 +0000 on 10 Nov (1320912604), Ian Campbell wrote:
> 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.

Yep.  Might be worth mentioning that HAP is called EPT and NPT (or RVI)
by the hardware vendors.

> > + * `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.

Yep. 

> > + * `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. 

Actually in the bad case the processes _do_ share address space
(otherwise the guest would be thrashing for RAM before it rnus out of
shadow memory).  Maybe 'a very large number of similar processes'?

Cheers,

Tim.

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