Xen devel - Jul 2008 - [PATCH] ioemu: drop unused 4MB video memory

ioemu: drop unused 4MB video memory

Since we only emulate the cirrus VGA video card which is only able to
expose 4MB video memory, we don''t need more than that.

Signed-off-by: Samuel Thibault <samuel.thibault@eu.citrix.com>

diff -r 7a918e567837 tools/ioemu/vl.h
--- a/tools/ioemu/vl.h	Thu Jul 10 15:34:44 2008 +0100
+++ b/tools/ioemu/vl.h	Thu Jul 10 17:09:37 2008 +0100
@@ -927,7 +927,7 @@
 
 /* vga.c */
 
-#define VGA_RAM_SIZE (8192 * 1024)
+#define VGA_RAM_SIZE (4096 * 1024)
 
 /* in ms */
 #define GUI_REFRESH_INTERVAL 30
diff -r 7a918e567837 tools/python/xen/xend/image.py
--- a/tools/python/xen/xend/image.py	Thu Jul 10 15:34:44 2008 +0100
+++ b/tools/python/xen/xend/image.py	Thu Jul 10 17:09:37 2008 +0100
@@ -833,8 +833,8 @@
         # buffer io page, buffer pio page and memmap info page
         extra_pages = 1024 + 5
         mem_kb += extra_pages * page_kb
-        # Add 8 MiB overhead for QEMU''s video RAM.
-        return mem_kb + 8192
+        # Add 4 MiB overhead for QEMU''s video RAM.
+        return mem_kb + 4096
 
     def getRequiredInitialReservation(self):
         return self.vm.getMemoryTarget()
@@ -876,8 +876,8 @@
         return rc
 
     def getRequiredAvailableMemory(self, mem_kb):
-        # Add 8 MiB overhead for QEMU''s video RAM.
-        return mem_kb + 8192
+        # Add 4 MiB overhead for QEMU''s video RAM.
+        return mem_kb + 4096
 
     def getRequiredInitialReservation(self):
         return self.vm.getMemoryTarget()

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Samuel Thibault writes ("[Xen-devel] [PATCH] ioemu: drop unused 4MB video
memory"):
> ioemu: drop unused 4MB video memory
> 
> Since we only emulate the cirrus VGA video card which is only able to
> expose 4MB video memory, we don''t need more than that.
I see this has gone into xen-unstable and also the new qemu-xen tree[1].

But it would be nice to understand the compatibility properties:
 * Evidently it breaks when tools have this change but qemu-dm
   doesn''t.  Is there a way to avoid this being a fatal error ?
 * I think I''m correct to assume that this change is harmless for
   a new qemu-dm and old tools ?
 * What about save/restore ?

Ian.

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iwj writes ("Re: [Xen-devel] [PATCH] ioemu: drop unused 4MB video
memory"):
> I see this has gone into xen-unstable and also the new qemu-xen tree[1].
[1] which apparently hasn''t been getting pushed properly which
I''m
investigating.

Ian.

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Hello,

Ian Jackson, le Mon 14 Jul 2008 10:35:15 +0100, a écrit
:
> Samuel Thibault writes ("[Xen-devel] [PATCH] ioemu: drop unused 4MB
video memory"):
> > ioemu: drop unused 4MB video memory
> > 
> > Since we only emulate the cirrus VGA video card which is only able to
> > expose 4MB video memory, we don''t need more than that.
> 
> I see this has gone into xen-unstable and also the new qemu-xen tree[1].
> 
> But it would be nice to understand the compatibility properties:
>  * Evidently it breaks when tools have this change but qemu-dm
>    doesn''t.
Yes.
> Is there a way to avoid this being a fatal error ?
Unfortunately VGA_RAM_SIZE is used in a lot of places of qemu so that''d
be a bit hard.
>  * I think I''m correct to assume that this change is harmless for
>    a new qemu-dm and old tools ?
Yes.
>  * What about save/restore ?
Ah, right, we need a compatibility version for that, I''ll work on it.

Samuel

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On 14/7/08 11:40, "Samuel Thibault"
<samuel.thibault@eu.citrix.com> wrote:
>>  * What about save/restore ?
> 
> Ah, right, we need a compatibility version for that, I''ll work on
it.
What''s the compatibility problem? Is VGA_RAM_SIZE baked into the qemu
save
image for some reason?

 -- Keir



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Keir Fraser, le Mon 14 Jul 2008 11:42:35 +0100, a écrit
:
> On 14/7/08 11:40, "Samuel Thibault"
<samuel.thibault@eu.citrix.com> wrote:
> 
> >>  * What about save/restore ?
> > 
> > Ah, right, we need a compatibility version for that, I''ll
work on it.
> 
> What''s the compatibility problem? Is VGA_RAM_SIZE baked into the
qemu save
> image for some reason?
The video memory is currently written by qemu yes.  Actually, if that
could be moved to libxc it would be some benefit, since that''s a
penalty
hit for live migration.

Samuel

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Samuel Thibault, le Mon 14 Jul 2008 11:49:33 +0100, a écrit
:
> Keir Fraser, le Mon 14 Jul 2008 11:42:35 +0100, a écrit :
> > On 14/7/08 11:40, "Samuel Thibault"
<samuel.thibault@eu.citrix.com> wrote:
> > 
> > >>  * What about save/restore ?
> > > 
> > > Ah, right, we need a compatibility version for that,
I''ll work on it.
> > 
> > What''s the compatibility problem? Is VGA_RAM_SIZE baked into
the qemu save
> > image for some reason?
> 
> The video memory is currently written by qemu yes.
Actually, the problem is also that everything (including qemu''s dirty
bitmap) gets initialized to 4MB before the load, and it''d be clumsy to
plumb a reallocation.  Bummer.  It''d be simpler to just go expense
these
4MB.

Samuel

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I believe we have this disabled currently, but there is code in upstream
qemu to support higher resolutions that actually make use of more memory
through stdvga + VBE bios exensions. It seems a bit counterproductive to put
in a lot of work to trim this down to 4 megs now if we''re eventually
going
to support the stdvga + VBE combo for higher resolutions. Or is this 4/8 meg
allocation stricly for cirrus, and stdvga mem allocation entirely separate?

/ Trolle

On Mon, Jul 14, 2008 at 12:09 PM, Samuel Thibault <
samuel.thibault@eu.citrix.com> wrote:
> Samuel Thibault, le Mon 14 Jul 2008 11:49:33 +0100, a écrit :
> > Keir Fraser, le Mon 14 Jul 2008 11:42:35 +0100, a écrit :
> > > On 14/7/08 11:40, "Samuel Thibault"
<samuel.thibault@eu.citrix.com>
> wrote:
> > >
> > > >>  * What about save/restore ?
> > > >
> > > > Ah, right, we need a compatibility version for that,
I''ll work on it.
> > >
> > > What''s the compatibility problem? Is VGA_RAM_SIZE baked
into the qemu
> save
> > > image for some reason?
> >
> > The video memory is currently written by qemu yes.
>
> Actually, the problem is also that everything (including qemu''s
dirty
> bitmap) gets initialized to 4MB before the load, and it''d be
clumsy to
> plumb a reallocation.  Bummer.  It''d be simpler to just go expense
these
> 4MB.
>
> Samuel
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel
>

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Trolle Selander, le Mon 14 Jul 2008 12:40:06 +0100, a écrit
:
> I believe we have this disabled currently, but there is code in upstream
qemu
> to support higher resolutions that actually make use of more memory through
> stdvga + VBE bios exensions. It seems a bit counterproductive to put in a
lot
> of work to trim this down to 4 megs now if we''re eventually going
to support
> the stdvga + VBE combo for higher resolutions.
Agreed.
> Or is this 4/8 meg allocation stricly for cirrus, and stdvga mem
> allocation entirely separate?
These allocations are independent, but are working the same way.

Samuel

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Samuel Thibault writes ("Re: [Xen-devel] [PATCH] ioemu: drop unused 4MB
video memory"):
> Ian Jackson, le Mon 14 Jul 2008 10:35:15 +0100, a écrit :
> > Is there a way to avoid this being a fatal error ?
> 
> Unfortunately VGA_RAM_SIZE is used in a lot of places of qemu so
that''d
> be a bit hard.
Err, you''re missing my point, which is that we want it to work with
old qemu and new hypervisor.  Obviously changing qemu now will not
allow us to do that ...
> >  * What about save/restore ?
> 
> Ah, right, we need a compatibility version for that, I''ll work on
it.
For the moment Keir and I have reverted this pair of changes.

Ian.

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Ian Jackson, le Mon 14 Jul 2008 13:53:39 +0100, a écrit
:
> Samuel Thibault writes ("Re: [Xen-devel] [PATCH] ioemu: drop unused
4MB video memory"):
> > Ian Jackson, le Mon 14 Jul 2008 10:35:15 +0100, a écrit :
> > > Is there a way to avoid this being a fatal error ?
> > 
> > Unfortunately VGA_RAM_SIZE is used in a lot of places of qemu so
that''d
> > be a bit hard.
> 
> Err, you''re missing my point, which is that we want it to work
with
> old qemu and new hypervisor.  Obviously changing qemu now will not
> allow us to do that ...
Oops, right.  No, there is no way.  Note however that the issue are with
the tools, not the hypervisor.

Samuel

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Samuel Thibault writes ("Re: [Xen-devel] [PATCH] ioemu: drop unused 4MB
video memory"):
> Ian Jackson, le Mon 14 Jul 2008 13:53:39 +0100, a écrit :
> > Err, you''re missing my point, which is that we want it to
work with
> > old qemu and new hypervisor.  Obviously changing qemu now will not
> > allow us to do that ...
> 
> Oops, right.  No, there is no way.  Note however that the issue are with
> the tools, not the hypervisor.
Yes, sorry, that''s what I meant.

Perhaps there should be some way for the tools to find out what fixes
or features qemu-dm has.

Ian.

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