Re: [PATCH updated v2] spapr: Fix EEH capability issue on KVM guest for PCI passthru

[David Gibson]

On Fri, May 14, 2021 at 12:03:10PM +1000, Oliver O'Halloran wrote:
> On Thu, May 13, 2021 at 2:22 PM David Gibson
> <david@gibson.dropbear.id.au> wrote:
> >
> > On Wed, May 05, 2021 at 08:18:27PM +0530, Mahesh Salgaonkar wrote:
> > > With upstream kernel, especially after commit 98ba956f6a389
> > > ("powerpc/pseries/eeh: Rework device EEH PE determination") we see that 
> > > KVM
> > > guest isn't able to enable EEH option for PCI pass-through devices 
> > > anymore.
> > >
> > > [root@atest-guest ~]# dmesg | grep EEH
> > > [    0.032337] EEH: pSeries platform initialized
> > > [    0.298207] EEH: No capable adapters found: recovery disabled.
> > > [root@atest-guest ~]#
> > >
> > > So far the linux kernel was assuming pe_config_addr equal to device's
> > > config_addr and using it to enable EEH on the PE through 
> > > ibm,set-eeh-option
> > > RTAS call. Which wasn't the correct way as per PAPR. The linux kernel
> > > commit 98ba956f6a389 fixed this flow. With that fixed, linux now uses PE
> > > config address returned by ibm,get-config-addr-info2 RTAS call to enable
> > > EEH option per-PE basis instead of per-device basis. However this has
> > > uncovered a bug in qemu where ibm,set-eeh-option is treating PE config
> > > address as per-device config address.
> >
> > Huh.  To be fair, the stuff about this in PAPR is nearly
> > incomprehensible, so we probably used what the kernel was doing as a
> > guide instead.
> 
> I found the PAPR documentation made some sense after I learned how EEH
> was handled on PCI(-X) systems. What's in Linux never made sense,
> unfortunately.

Indeed.

> > Hmm.. shouldn't we at least check that the supplied config_addr
> > matches the one it should be for this PHB, rather than just ignoring
> > it?
> 
> I think that'd cause issues with older kernels.

Oh, good point.

> Prior to the rework
> mentioned by Mahesh (linux commit 98ba956f6a389 ("powerpc/pseries/eeh:
> Rework device EEH PE determination")) the kernel would call
> eeh-set-option for each device in the PE using the device's
> config_address as the argument rather than the PE address. If we
> return an error from eeh-set-option when the argument isn't a valid PE
> address then older kernels will interpret that as EEH not being
> supported. That really needs to be called out in a comment though.
> Preferably with kernel version numbers, etc.

Agreed.

> > ..and, looking back at rtas_ibm_get_config_addr_info2(), I think
> > that looks wrong in the case of PCI bridges.  AFAICT it gives an
> > address that depends on the bus, but in other places we assume that
> > the entire PHB is a single PE on the guest side, so it really
> > shouldn't.
> 
> Yep, get_config_addr_info2 should map every device inside that PE to
> the same PE address, even when they're on child busses.

Right.

> That said, I'm
> not sure how well EEH works when there's a mix of real (vfio) and
> emulated (qemu bridges) devices in the same PHB.

I think it'll kind of work, as long as there's only real devices from
a single host PE on there.  The emulated devices will basically just
ignore EEH, but I think they should still apply ok to the passthrough
devices.

> Can VFIO pass through
> a bridge?

I don't think so.

-- 
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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