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

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Message-ID: <86a5qobkt8.wl-maz@kernel.org>
Date:   Tue, 05 Dec 2023 17:50:27 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Jason Gunthorpe <jgg@...dia.com>, 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,
        lpieralisi@...nel.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, 05 Dec 2023 17:33:01 +0000,
Catalin Marinas <catalin.marinas@....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@...dia.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.