Re: [PATCH v2 1/1] KVM: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory

From: Jason Gunthorpe <jgg@nvidia.com>
To: Lorenzo Pieralisi <lpieralisi@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Marc Zyngier <maz@kernel.org>,
	ankita@nvidia.com,
	Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>,
	oliver.upton@linux.dev, suzuki.poulose@arm.com,
	yuzenghui@huawei.com, will@kernel.org, ardb@kernel.org,
	akpm@linux-foundation.org, gshan@redhat.com, aniketa@nvidia.com,
	cjia@nvidia.com, kwankhede@nvidia.com, targupta@nvidia.com,
	vsethi@nvidia.com, acurrid@nvidia.com, apopple@nvidia.com,
	jhubbard@nvidia.com, danw@nvidia.com, mochs@nvidia.com,
	kvmarm@lists.linux.dev, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 1/1] KVM: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory
Date: Tue, 5 Dec 2023 10:44:17 -0400	[thread overview]
Message-ID: <20231205144417.GE2692119@nvidia.com> (raw)
In-Reply-To: <ZW81mT4WqKqtLnid@lpieralisi>

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.

Jason