linux-kernel - Re: [PATCH v2 1/1] KVM: arm64: allow the VM to select DEVICE_* and NORMAL

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Message-ID: <ZW9OtrUa_fPA0_Ns@arm.com>
Date:   Tue, 5 Dec 2023 16:24:22 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Marc Zyngier <maz@...nel.org>, ankita@...dia.com,
        Shameerali Kolothum Thodi 
        <shameerali.kolothum.thodi@...wei.com>, oliver.upton@...ux.dev,
        suzuki.poulose@....com, yuzenghui@...wei.com, will@...nel.org,
        ardb@...nel.org, akpm@...ux-foundation.org, gshan@...hat.com,
        aniketa@...dia.com, cjia@...dia.com, kwankhede@...dia.com,
        targupta@...dia.com, vsethi@...dia.com, acurrid@...dia.com,
        apopple@...dia.com, jhubbard@...dia.com, danw@...dia.com,
        mochs@...dia.com, kvmarm@...ts.linux.dev, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/1] KVM: arm64: allow the VM to select DEVICE_* and
 NORMAL_NC for IO memory

On Tue, Dec 05, 2023 at 10:44:17AM -0400, Jason Gunthorpe wrote:
> On Tue, Dec 05, 2023 at 03:37:13PM +0100, Lorenzo Pieralisi wrote:
> > On Tue, Dec 05, 2023 at 09:05:17AM -0400, Jason Gunthorpe wrote:
> > > On Tue, Dec 05, 2023 at 11:40:47AM +0000, Catalin Marinas wrote:
> > > > > - Will had unanswered questions in another part of the thread:
> > > > > 
> > > > >   https://lore.kernel.org/all/20231013092954.GB13524@willie-the-truck/
> > > > > 
> > > > >   Can someone please help concluding it?
> > > > 
> > > > Is this about reclaiming the device? I think we concluded that we can't
> > > > generalise this beyond PCIe, though not sure there was any formal
> > > > statement to that thread. The other point Will had was around stating
> > > > in the commit message why we only relax this to Normal NC. I haven't
> > > > checked the commit message yet, it needs careful reading ;).
> > > 
> > > Not quite, we said reclaiming is VFIO's problem and if VFIO can't
> > > reliably reclaim a device it shouldn't create it in the first place.
> > 
> > I think that as far as device reclaiming was concerned the question
> > posed was related to memory attributes of transactions for guest
> > mappings and the related grouping/ordering with device reset MMIO
> > transactions - it was not (or wasn't only) about error containment.
> 
> Yes. It is VFIO that issues the reset, it is VFIO that must provide
> the ordering under the assumption that NORMAL_NC was used.

And does it? Because VFIO so far only assumes Device-nGnRnE. Do we need
to address this first before attempting to change KVM? Sorry, just
questions, trying to clear the roadblocks.

-- 
Catalin