Re: [Qemu-devel] spapr_pci: Advertise BAR reallocation capability

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Jun 13, 2019 at 11:37:45AM +1000, Alexey Kardashevskiy wrote:
> 
> 
> On 12/06/2019 16:11, David Gibson wrote:
> > On Thu, Jun 06, 2019 at 02:09:19PM +1000, Alexey Kardashevskiy wrote:
> >> The pseries guests do not normally allocate PCI resouces and rely on
> >> the system firmware doing so. Furthermore at least at some point in
> >> the past the pseries guests won't even be allowed to change BARs, probably
> >> it is still the case for phyp. So since the initial commit we have [1]
> >> which prevents resource reallocation.
> >>
> >> This is not a problem until we want specific BAR alignments, for example,
> >> PAGE_SIZE==64k to make sure we can still map MMIO BARs directly. For
> >> the boot time devices we handle this in SLOF [2] but since QEMU's RTAS
> >> does not allocate BARs, the guest does this instead and does not align
> >> BARs even if Linux is given pci=resource_alignment=16@pci:0:0 as
> >> PCI_PROBE_ONLY makes Linux ignore alignment requests.
> >>
> >> ARM folks added a dial to control PCI_PROBE_ONLY via the device tree [3].
> >> This makes use of the dial to advertise to the guest that we can handle
> >> BAR reassignments.
> >>
> >> We do not remove the flag from [1] as pseries guests are still supported
> >> under phyp so having that removed may cause problems.
> >>
> >> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/platforms/pseries/setup.c?h=v5.1#n773
> >> [2] https://git.qemu.org/?p=SLOF.git;a=blob;f=board-qemu/slof/pci-phb.fs;h=06729bcf77a0d4e900c527adcd9befe2a269f65d;hb=HEAD#l338
> >> [3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f81c11af
> >> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> > 
> > Changing a guest visible property, that could have a big effect on how
> > the guest behaves, without a machine version change seems... unwise.
> 
> 
> As a general rule - sure, not good. In this particular case QEMU has
> always been able to cope with BAR reallocations.

That's not really the point.  What I'm worried about is some old
kernel version running on a guest in the wild having a bug here and
the supposedly compatible qemu change breaking it.

> What could probably
> make sense is having it as a machine option (pci-probe-only=off by
> default) in case if we find some old kernel which cannot handle
> "linux,pci-probe-only" but I seriously doubt we'll find such a broken
> kernel - I do remove the probe-only switch from guest kernels on a
> regular basis last 7 or so years when debugging.

Yeah, doing it when debugging isn't really the same as exercising it
in production environments.

-- 
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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