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Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-bel
From: |
Eduardo Habkost |
Subject: |
Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option |
Date: |
Wed, 20 Jan 2016 13:34:29 -0200 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Wed, Jan 20, 2016 at 03:55:33PM +0100, Gerd Hoffmann wrote:
> Hi,
>
> > > + * qemu -M pc,max-ram-below-4g=2G -m 4G -> 2048M low, 2048M
> > > high
>
> > I assume max-ram-below-4g > 3.5G was unsupported before, and we
> > are breaking compatibility intentionally.
>
> max-ram-below-4g did only reduce memory, so max-ram-below-4g > 3.5G (or
> max-ram-below-4g > 3G with gigabyte align) had no effect and therefore
> is something pretty pointless.
I see. So, if max-ram-below-4g > 3G with gigabyte align was
unsupported too, my example below is also not relevant regarding
compatibility.
>
> I'd expect the one case quoted above to be the only case relevant in
> practice (i.e. move split from 3G to 2G for more PCI I/O space),
> especially given that the option was added after gigabyte alignment
> support.
>
> > Because this patch also
> > changes the resulting memory layout when
> > 3G < max_ram_below_4g < ram_size < 3.5G
> > e.g.:
> > qemu -M pc,max-ram-below-4g=3200M -m 3328M
>
> Ah, I see. With the patch applied the gigabyte align option is weighed
> higher, so qemu wouldn't give you 3200M lowmem. I would be highly
> surprised to see such a configuration in the wild ...
The new behavior looks OK, considering that the option is just
"_max_ RAM below 4G", not "split lowmem at this exact address".
But it is inconsistent with the behavior when
max-ram-below-4g < 3G (where gigabyte_align is ignored), and
something we will need to keep compatibility for a long time.
Considering that we never supported
gigabyte_align && max_ram_below_4g > 3G ||
max_ram_below_4g > 3.5G
before, we could simply remove the MachineClass::gigabyte_align
field from pc_piix, and just do the following:
* pc > 1.7: max_ram_below_4g = 3G
(equivalent to gigabyte_align=true)
* pc <= 1.7: max_ram_below_4g = 3.5G
(equivalent to gigabyte_align=false)
--
Eduardo
- [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Gerd Hoffmann, 2016/01/08
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Marcel Apfelbaum, 2016/01/11
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Eric Blake, 2016/01/14
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Eduardo Habkost, 2016/01/19
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Gerd Hoffmann, 2016/01/20
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option,
Eduardo Habkost <=
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Eduardo Habkost, 2016/01/20
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Michael S. Tsirkin, 2016/01/20
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Gerd Hoffmann, 2016/01/21
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Michael S. Tsirkin, 2016/01/21
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Gerd Hoffmann, 2016/01/22
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Michael S. Tsirkin, 2016/01/24