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: Tue, 05 Dec 2023 17:50:27 +0000	[thread overview]
Message-ID: <86a5qobkt8.wl-maz@kernel.org> (raw)
In-Reply-To: <ZW9ezSGSDIvv5MsQ@arm.com>

On Tue, 05 Dec 2023 17:33:01 +0000,
Catalin Marinas <catalin.marinas@arm.com> wrote:
> 
> On Tue, Dec 05, 2023 at 05:01:28PM +0000, Marc Zyngier wrote:
> > On Tue, 05 Dec 2023 16:43:18 +0000,
> > Jason Gunthorpe <jgg@nvidia.com> wrote:
> > > On Tue, Dec 05, 2023 at 04:22:33PM +0000, Catalin Marinas wrote:
> > > > That's an argument to restrict this feature to PCIe. It's really about
> > > > fewer arguments on the behaviour of other devices. Marc did raise
> > > > another issue with the GIC VCPU interface (does this even have a vma in
> > > > the host VMM?). That's a class of devices where the mapping is
> > > > context-switched, so the TLBI+DSB rules don't help.
> > 
> > There is no vma. The CPU interface is entirely under control of KVM.
> > Userspace only provides the IPA for the mapping.
> 
> That's good to know. We can solve the GIC issue by limiting the
> relaxation to those mappings that have a user vma.

Yes, this should definitely be part of the decision.

> 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.

	M.

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