Re: [Qemu-devel] [PATCH qemu] vmstate: Define VARRAY with VMS_ALLOC

[David Gibson]

On Wed, Mar 16, 2016 at 09:07:59AM +0000, Dr. David Alan Gilbert wrote:
> * David Gibson (address@hidden) wrote:
> > On Tue, Mar 15, 2016 at 12:01:04PM +0000, Dr. David Alan Gilbert wrote:
> > > * Alexey Kardashevskiy (address@hidden) wrote:
> > > > This allows dynamic allocation for migrating arrays.
> > > > 
> > > > Already existing VMSTATE_VARRAY_UINT32 requires an array to be
> > > > pre-allocated, however there are cases when the size is not known in
> > > > advance and there is no real need to enforce it.
> > > > 
> > > > This defines another variant of VMSTATE_VARRAY_UINT32 with WMS_ALLOC
> > > > flag which tells the receiving side to allocate memory for the array
> > > > before receiving the data.
> > > > 
> > > > The first user of it is the "pseries" machine (POWER8) with
> > > > dynamic DMA windows which existence and size are totally dynamic.
> > > 
> > > You say totally dynamic, how big do they get out of interest?
> > 
> > They're basically used to map all guest RAM.  Typically we'd be
> > looking at one 64-bit TCE per 64K guest page, so we'd be looking at
> > 1/8192th of RAM size.
> > 
> > Since we can in theory have guests in the 1T+ range, that might start
> > getting pretty big, so we probably should look at incremental transfer
> > of the TCE tables at some point.
> 
> OK, you probably need to bump up MAX_VM_CMD_PACKAGED_SIZE (sysemu.h)
> which is an arbitrary size limit for postcopies device data; it's currently
> 16MB.  It's just there to stop anything silly, so just turn it up a bit.
> 
> However, if it is fixed at one 64-bit TCE per guest page, why do you
> need to dynamically allocate it during migration load, can't you
> statically allocate once you know guests memory size?

So, guest memory size is what it will be in practice (with Linux
guests), but that's under the guest's control, not ours; at least in
theory it can be different.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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