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Re: [Xen-devel] [PATCH] x86: set dom0's default maximum reservation to the initial number of pages



>>> On 20.03.12 at 19:21, David Vrabel <david.vrabel@xxxxxxxxxx> wrote:
> From: David Vrabel <david.vrabel@xxxxxxxxxx>
> 
> If a maximum reservation for dom0 is not explictly given (i.e., no
> dom0_mem=max:MMM command line option), then set the maximum
> reservation to the initial number of pages.  This is what most people
> seem to expect when they specify dom0_mem=512M (i.e., exactly 512 MB
> and no more).

I think I had seen this before (not sure whether it came from you),
and had already NAK-ed the idea back then. Please let's not play
with the meaning of existing hypervisor options, unless the "most"
in your description can validly be replaced by "all". In this case, _I_
am using the option in its current sense almost everywhere
(expecting to be able to balloon up beyond the initial allocation),
and it can also be useful in hotplug capable machines. If someone
wants a particular maximum, (s)he should add the respective
command line option.

> This change means that with Linux 3.0.5 and later kernels,
> dom0_mem=512M has the same result as older, 'classic Xen' kernels. The
> older kernels used the initial number of pages to set the maximum
> number of pages and did not query the hypervisor for the maximum
> reservation.

If the pv-ops kernels behave differently in this aspect than the forward
ported ones, then it should really be the newer one to get adjusted to
the original behavior (if so intended), unless the old behavior was
completely insane. Adjusting the hypervisor for a shortcoming(?) in a
particular implementation of a Dom0 kernel doesn't seem to be the right
route - please also consider non-Linux Dom0 kernels that might expect
the current behavior.

> It is still possible to have a larger reservation by explicitly
> specifying dom0_mem=max:MMM.

I am not intending to update dozens of command lines.

Jan

> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> ---
> Keir,
> 
> Suggest waiting for an Ack from Konrad (I think it results in the
> behaviour we want but would prefer it if Konrad confirmed).
> 
> Also consider for 4.1.
> 
> Thanks.
> 
> David
> 
>  docs/misc/xen-command-line.markdown |    8 +++++++-
>  xen/arch/x86/domain_build.c         |   10 ++++++++++
>  2 files changed, 17 insertions(+), 1 deletions(-)
> 
> diff --git a/docs/misc/xen-command-line.markdown 
> b/docs/misc/xen-command-line.markdown
> index beb8462..0798700 100644
> --- a/docs/misc/xen-command-line.markdown
> +++ b/docs/misc/xen-command-line.markdown
> @@ -221,12 +221,18 @@ Specify the total size for dom0.
>  ### dom0\_mem (x86)
>  > `= List of ( min:<value> | max: <value> | <value> )`
>  
> -each `<value>` is a size parameter.  If the size is positive, it represents 
> an 
> absolute value.  If the size is negative, the size specified is subtracted 
> from the total available memory.
> +Specify the amount of memory for the initial domain (dom0) and the maximum 
> reservation (the maximum amount of memory that dom0 can be increased or 
> ballooned to).
> +
> +Each `<value>` is a size parameter.  If the size is positive, it represents 
> an absolute value.  If the size is negative, the size specified is subtracted 
> from the total available memory.
>  
>  * `min:<value>` specifies the minimum amount of memory allocated to dom0.
>  * `max:<value>` specifies the maximum amount of memory allocated to dom0.
>  * `<value>` specified the exact amount of memory allocated to dom0.
>  
> +If `max:<value>` is specified then this sets the maximum reservation, 
> otherwise the maximum reservation is set to the amount of memory allocated to 
> dom0.
> +
> +For example, with `dom0_mem=512M`, dom0 starts with 512 MB and cannot 
> balloon up any more. With `dom0_mem=512M,max:2G`, dom0 starts with 512 MB of 
> memory and can balloon up to 2 GB.
> +
>  ### dom0\_shadow
>  ### dom0\_vcpus\_pin
>  > `= <boolean>`
> diff --git a/xen/arch/x86/domain_build.c b/xen/arch/x86/domain_build.c
> index b3c5d4c..0c09abc 100644
> --- a/xen/arch/x86/domain_build.c
> +++ b/xen/arch/x86/domain_build.c
> @@ -253,6 +253,16 @@ static unsigned long __init compute_dom0_nr_pages(
>      }
>  #endif
>  
> +    /*
> +     * Set dom0's maximum reservation.
> +     *
> +     * If no maximum was set with dom0_mem=max:MMM, then the maximum
> +     * is the same as the initial number of pages.  This is so
> +     * dom0_mem=MMM gives the behaviour most people expect (i.e., this
> +     * much RAM and no more).
> +     */
> +    if ( max_pages == LONG_MAX )
> +        max_pages = nr_pages;
>      d->max_pages = min_t(unsigned long, max_pages, UINT_MAX);
>  
>      return nr_pages;
> -- 
> 1.7.2.5
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxx 
> http://lists.xen.org/xen-devel 



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