ppc kvm: support of 64K guest page size with 4K host pagesize?

ppc kvm: support of 64K guest page size with 4K host pagesize?

[Michael Tokarev]

There's a bug report filed in debian against qemu,
https://bugs.debian.org/1107288 - saying kvm on ppc does not
work on debian (anymore) due to qemu requesting unrealistic
(non-existing) page size of 64K on a host which only has 4K.

I don't know much about PPC, - what's the issue here? Should
qemu use the same page size for kvm as on the host?

Thanks,

/mjt

Re: ppc kvm: support of 64K guest page size with 4K host pagesize?

[Thomas Huth]

On 05/06/2025 08.34, Michael Tokarev wrote:
> There's a bug report filed in debian against qemu,
> https://bugs.debian.org/1107288 - saying kvm on ppc does not
> work on debian (anymore) due to qemu requesting unrealistic
> (non-existing) page size of 64K on a host which only has 4K.
> 
> I don't know much about PPC, - what's the issue here? Should
> qemu use the same page size for kvm as on the host?

Looking at

  https://gitlab.com/qemu-project/qemu/-/commit/2309832afdaf8d6451ebc2e81bace8eb8ea41293

it seems like this was done on purpose? David, do you remember why 
auto-detecting was not a good idea here?

Anyway, seems like there is a hpt-max-page-size property that could be used 
to set the value manually - maybe you could suggest that to the user as a 
work-around?

  Thomas

Re: ppc kvm: support of 64K guest page size with 4K host pagesize?

[David Gibson]

[-- Attachment #1: Type: text/plain, Size: 1555 bytes --]

On Thu, Jun 05, 2025 at 08:52:44AM +0200, Thomas Huth wrote:
> On 05/06/2025 08.34, Michael Tokarev wrote:
> > There's a bug report filed in debian against qemu,
> > https://bugs.debian.org/1107288 - saying kvm on ppc does not
> > work on debian (anymore) due to qemu requesting unrealistic
> > (non-existing) page size of 64K on a host which only has 4K.
> > 
> > I don't know much about PPC, - what's the issue here? Should
> > qemu use the same page size for kvm as on the host?
> 
> Looking at
> 
>  https://gitlab.com/qemu-project/qemu/-/commit/2309832afdaf8d6451ebc2e81bace8eb8ea41293
> 
> it seems like this was done on purpose? David, do you remember why
> auto-detecting was not a good idea here?

Because the available page sizes are guest visible.  So if we
auto-detected, guests could be silently migration-incompatible with
the same command line.

> Anyway, seems like there is a hpt-max-page-size property that could be used
> to set the value manually - maybe you could suggest that to the user as a
> work-around?

Yes, that's what you'll need to do.  We set the default to 64kiB
pages, since at that point 64kiB was the default for all major host
distros, and guests benefit significantly from being able to use 64kiB
pages.  If you want guests compatible with 4kiB page hosts, you need
to use this parameter.

-- 
David Gibson (he or they)	| 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

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]