Re: [Qemu-devel] [PATCH 0/5] pc: make ROMs resizeable

[Michael S. Tsirkin]

On Wed, Nov 19, 2014 at 01:01:14PM +0530, Amit Shah wrote:
> On (Mon) 17 Nov 2014 [22:08:46], Michael S. Tsirkin wrote:
> > At the moment we migrate ROMs which reside in fw cfg, which allows
> > changing ROM code at will, and supports migrating largish blocks early,
> > with good performance.
> > However, we are running into a problem: changing size breaks
> > migration every time.
> > This already requires somewhat messy compatibility support in
> > acpi generation code, and it looks like there'll be more to come.
> > 
> > Rather than try to guess the correct size once and for all,
> > this patchset tries to make code future-proof, by
> > adding support for resizeable ram blocks.
> > 
> > A (possibly very high) amount of space in ram_addr_t space is reserved
> > for each block, but never allocated.
> > If incoming block size differs from current size, block is
> > reallocated. FW CFG is also notified and updated accordingly.
> > 
> > To simplify things, I didn't add support for resizing
> > actual RAM: device RAM such as fw cfg ROMs are never mapped
> > into guests directly, so instead I added an API to
> > flag device RAM explicitly, and manage them using
> > simple alloc/free/realloc
> > 
> > Considering this promises to rid us of worries about ROM size considerations
> > once and for all, I thinking about pushing this as a "kind of bugfix" before
> > 2.2, so we don't need to maintain more band-aids in 2.3 and on.
> 
> I'd rather wait for 2.3; we've done this for a couple of releases
> already, so what's one more.  And we're at rc2 already..

Paolo feels the same, and I agree.

> > Note: migration stream is unaffected by these patches.
> > This makes it possible to enable this functionality
> > unconditionally, for all machine types.
> > 
> > In the future, this might be handy for other things,
> > such as changing kernels loaded on command line
> > across migrations.
> 
> I think that'll be too risky; unless we do S4 before / after
> migration to ensure the kernel realises things might be changing
> beneath its feet.
> 
>               Amit

Well - guest never sees the resizing. It happens before we start the VM.
So I don't see the issue - could you clarify please?

-- 
MST