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

From: Lorenzo Pieralisi <lpieralisi@kernel.org>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>, Jason Gunthorpe <jgg@nvidia.com>,
	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: Wed, 6 Dec 2023 12:52:22 +0100	[thread overview]
Message-ID: <ZXBgdnz7wVYuDCgU@lpieralisi> (raw)
In-Reply-To: <ZW9uqu7yOtyZfmvC@arm.com>

On Tue, Dec 05, 2023 at 06:40:42PM +0000, Catalin Marinas wrote:
> On Tue, Dec 05, 2023 at 05:50:27PM +0000, Marc Zyngier wrote:
> > On Tue, 05 Dec 2023 17:33:01 +0000,
> > Catalin Marinas <catalin.marinas@arm.com> wrote:
> > > Ideally we should do this for vfio only but we don't have an easy
> > > way to convey this to KVM.
> > 
> > But if we want to limit this to PCIe, we'll have to find out. The
> > initial proposal (a long while ago) had a flag conveying some
> > information, and I'd definitely feel more confident having something
> > like that.
> 
> We can add a VM_PCI_IO in the high vma flags to be set by
> vfio_pci_core_mmap(), though it limits it to 64-bit architectures. KVM
> knows this is PCI and relaxes things a bit. It's not generic though if
> we need this later for something else.
> 
> A question for Lorenzo: do these BARs appear in iomem_resource? We could
> search that up instead of a flag, something like the page_is_ram()
> helper.

They do but an iomem_resource look-up alone if I am not mistaken would
not tell us if it is PCI address space, we would need additional checks
(like detecting if we are decoding an address within a PCI host bridge
windows) to determine that if we go down this route.

Lorenzo