linux-kernel - Re: [PATCH v6 0/9] vITS Migration fixes and reset

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Date:   Tue, 31 Oct 2017 07:43:13 +0100
From:   Christoffer Dall <cdall@...aro.org>
To:     Auger Eric <eric.auger@...hat.com>
Cc:     eric.auger.pro@...il.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu,
        marc.zyngier@....com, peter.maydell@...aro.org,
        andre.przywara@....com, wanghaibin.wang@...wei.com,
        wu.wubin@...wei.com, drjones@...hat.com, wei@...hat.com
Subject: Re: [PATCH v6 0/9] vITS Migration fixes and reset

On Mon, Oct 30, 2017 at 08:59:36AM +0100, Auger Eric wrote:
> Hi Christoffer,
> 
> On 30/10/2017 07:20, Christoffer Dall wrote:
> > Hi Eric,
> > 
> > On Thu, Oct 26, 2017 at 05:23:02PM +0200, Eric Auger wrote:
> >> This series fixes various bugs observed when saving/restoring the
> >> ITS state before the guest writes the ITS registers (on first boot or
> >> after reset/reboot).
> >>
> >> This is a follow up of Wanghaibin's series [1] plus additional
> >> patches following additional code review. It also proposes one
> >> ITS reset implementation.
> >>
> >> Currently, the in-kernel emulated ITS is not reset. After a
> >> reset/reboot, the ITS register values and caches are left
> >> unchanged. Registers may point to some tables in guest memory
> >> which do not exist anymore. If an ITS state backup is initiated
> >> before the guest re-writes the registers, the save fails
> >> because inconsistencies are detected. Also restore of data saved
> >> as such moment is failing.
> >>
> >> Patches [1-4] are fixes of bugs observed during migration at
> >> early guets boot stage.
> >> - handle case where all collection, device and ITT entries are
> >>   invalid on restore (which is not an error)
> >> - Check the GITS_BASER<n> valid bit before attempting the save
> >>   any table
> >> - Check the GITS_BASER<n> and GITS_CBASER are valid before enabling
> >>   the ITS
> >>
> >> Patches [5-9] allow to empty the caches on reset and implement a
> >> new ITS reset IOCTL
> > 
> > I applied patches 1-4 to kvmarm/master and included them in a late pull
> > request to kvm.
> > 
> > I also took the remaining patches with the adjusted comment in
> > kvmarm/next.
> 
> OK Thanks.
> > 
> > One question:  Don't we have a remaining issue to support saving the
> > collection table even if the device table is inconsistent and vice
> > versa?  Are you planning on picking up that work?
> 
> Actually to make it clear, patches 1-4 don't fix all failures but we
> discussed at KVM forum that we shouldn't try to fix all of them without
> a proper ITS reset implementation. So indeed even with patches 1-4 you
> can get the migration failing as the save can happen after a reset,
> in-between the collection creation and the PCIe device MSI registration.
> This happens because the caches are not void and the L1 device table
> entries are not valid. In that case the device table save fails and we
> get no chance to save the collection as we abort the save immediately.
> So on restore, guest will not work properly.
> 
> But on top of kvmarm/next, caches are void and we shouldn't face this
> issue anymore. So in case the device table save fails, I think it still
> makes sense to return an error.
> 
> But this means that the migration of ITS at early guest boot stage only
> is fixed on kvmarm/next.
> 

I think you're right.

Thanks,
-Christoffer