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

From: Marc Zyngier <maz@kernel.org>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: 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,
	lpieralisi@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, 06 Dec 2023 11:39:03 +0000	[thread overview]
Message-ID: <868r67blwo.wl-maz@kernel.org> (raw)
In-Reply-To: <ZW9uqu7yOtyZfmvC@arm.com>

On Tue, 05 Dec 2023 18:40:42 +0000,
Catalin Marinas <catalin.marinas@arm.com> 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.

Either that, or something actually describing the attributes that VFIO
wants.

And I very much want it to be a buy-in behaviour, not something that
automagically happens and changes the default behaviour for everyone
based on some hand-wavy assertions.

If that means a userspace change, fine by me. The VMM better know what
is happening.

	M.

-- 
Without deviation from the norm, progress is not possible.