Re: [Xen-devel] [PATCH v6 0/3] Add max-ram-below-4g (was Add pci_hole_min_size machine option)

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

On Tue, Jun 17, 2014 at 02:32:07PM -0400, Don Slutz wrote:
> Changes v5 to v6:
>   rebased on git://git.kernel.org/pub/scm/virt/kvm/mst/qemu.git pci
> 
>   Changed default handling.  Most pc machines are now set to 3.5G
>   (0xe0000000) and q35 are set to 2.75G (0xb0000000).  The special
>   value of 0 is used to flag the complex gigabyte_align memory
>   layout selection.
> 
>   I did change the default for pc-i440fx-2.1 to 3G and pc-q35-2.1 to
>   2G instead of the complex gigabyte_align.  This looks ok to me
>   because the statement by Gerd Hoffmann is about more ram for 32
>   bit (+non-PAE) guests can now have more then the defualt by
>   specifing the new option like "max-ram-below-4g=3.75G" or
>   "max-ram-below-4g=3.9375G".  Or if that is too complex to
>   understand just select pc-i440fx-2.0 or pc-q35-2.0 to get what
>   they use to get.

I don't think it's OK.

The beauty of Gerd's hack is that it does the right thing
for 32 bit guests without any need for tweaks.
I.e. most users get good performance.

By comparison this setting seems easier to get wrong than right.

And we definitely don't want to push people to use 2.0
compat setting unless they need bug for bug compatible hardware.

Generally, I'm still confused about this feature.
It seems to be about helping 32 bit non PAE guests, is that right?
But if that's the case why use >4G memory?


>   Michael S. Tsirkin, Igor Mammedov, Marcel Apfelbaum:
>     #2 "pc & q35: Add new machine opt max-ram-below-4g":
>        Added setting of .default_machine_opts to include max-ram-below-4g
>          for all pc types.
>        Removed gigabyte_align.
>        Added warning on small value.
>        "less then" to "less than"
> 
>   #3 "xen-hvm: Pass is_default to xen_hvm_init":
>     Changed to work with the changes in #2:
>       Added max_ram_below_4g_changed in order to know if it is a default value
>       or a user specified one.
>     Added "assert(pc_machine->max_ram_below_4g_changed > 0)" so that
>       "make check" will abort on default not set for any of the pc or
>       q35 machine types.
>     Dropped "Acked-by: Stefano Stabellin" do to bigger change.
> 
> 
> Changes v4 to v5:
>   Re-work based on:
> 
>   https://github.com/imammedo/qemu/commits/memory-hotplug-v11
> 
>   And so it now depends on this patch set.
> 
>   Stefano Stabellini:
>     #3 "xen-hvm: Pass is_default to xen_hvm_init"
>       Acked-by 
>       Minor change of pmc to pcms.
> 
> Changes v3 to v4:
>   Split out #2 "GlobalProperty: Display warning about unused -global"
>   rebase on e00fcfe (origin/master)
>   rename xen-all to xen-hvm
> 
>   Adjust #1 "xen-hvm: Fix xen_hvm_init() to adjust pc memory layout"
>     Switch Acked-by & Signed-off-by
>     rebase on master
> 
>   Rework #3 "xen-hvm: Pass is_default to xen_hvm_init":
>     To pass is_default instead of max_ram_below_4g.
>     Also did not add "Acked-by: Stefano Stabellini" since code changed a lot.
> 
>   Andreas Färber:
>     all: Remove dot at end of subject
>     #3 "xen-hvm: Pass is_default to xen_hvm_init"
>       Adjust comment formatting.
> 
>   Andreas Färber, Paolo Bonzini, Marcel Apfelbaum:
>     rework to use "opts per machine"
>     Drop old #3, new #2.
>   
> 
> Changes v2 to v3:
>   Stefano Stabellini:
>     Acked-by #1 "xen-all: Fix xen_hvm_init() to adjust pc memory"
>     Adjust for code readability #4 "xen-all: Pass max_ram_below_4g to 
> xen_hvm_init."
>        Set max_ram_below_4g always and use it to calculate above_4g_mem_size,
>        below_4g_mem_size.
> 
> Changes v1 to v2:
>   Michael S. Tsirkin:
>     Rename option.
>     Only add it to machine types that support it.
>   Split into 4 parts.
> 
> 1/4 -- xen-all: Fix xen_hvm_init() to adjust pc memory layout
> 
>   This looks to be a possible bug that has yet to be found.
>   below_4g_mem_size and above_4g_mem_size are stored in PcGuestInfo
>   (pc_guest_info_init) which are currently not "correct".  This and
>   4/4 change the same lines.
> 
> 2/4 -- GlobalProperty: Display warning about unused -global
> 
>     My testing showed that setting a global property on an object
>     that is not used is not reported at all.  This is added to help
>     when the new global is set but not used.  The negative not_used
>     was picked so that all static objects are assumed to be used
>     even when they are not.
> 
> 3/4 -- pc & q35: Add new object pc-memory-layout
> 
>   The objects that it might make sense to add this property to all
>   get created too late.  So add a new object just to hold this
>   property.  Name it so that it is expected that only pc (and q35)
>   machine types support it.
> 
> 4/4 -- xen-all: Pass max_ram_below_4g to xen_hvm_init
> 
>   Seprate the xen only part of the change.  Currectly based on patch 1/4
> 
> Don Slutz (3):
>   xen-hvm: Fix xen_hvm_init() to adjust pc memory layout
>   pc & q35: Add new machine opt max-ram-below-4g
>   xen-hvm: Pass is_default to xen_hvm_init
> 
>  hw/i386/pc.c         | 45 ++++++++++++++++++++++++++++++++
>  hw/i386/pc_piix.c    | 73 
> +++++++++++++++++++++++++++++++++-------------------
>  hw/i386/pc_q35.c     | 72 ++++++++++++++++++++++++++++++++-------------------
>  include/hw/i386/pc.h |  4 +++
>  include/hw/xen/xen.h |  3 ++-
>  vl.c                 |  4 +++
>  xen-hvm-stub.c       |  3 ++-
>  xen-hvm.c            | 46 +++++++++++++++++++--------------
>  8 files changed, 177 insertions(+), 73 deletions(-)
> 
> -- 
> 1.8.4

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