linux-kernel - Re: [RFC PATCH 08/12] vfio/pci: Create host unaccessible dma-buf for private device

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Message-ID: <Z36ulpCoJAllp4fP@yilunxu-OptiPlex-7050>
Date: Thu, 9 Jan 2025 00:57:58 +0800
From: Xu Yilun <yilun.xu@...ux.intel.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: kvm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
	sumit.semwal@...aro.org, christian.koenig@....com,
	pbonzini@...hat.com, seanjc@...gle.com, alex.williamson@...hat.com,
	vivek.kasireddy@...el.com, dan.j.williams@...el.com, aik@....com,
	yilun.xu@...el.com, linux-coco@...ts.linux.dev,
	linux-kernel@...r.kernel.org, lukas@...ner.de, yan.y.zhao@...el.com,
	daniel.vetter@...ll.ch, leon@...nel.org, baolu.lu@...ux.intel.com,
	zhenzhong.duan@...el.com, tao1.su@...el.com
Subject: Re: [RFC PATCH 08/12] vfio/pci: Create host unaccessible dma-buf for
 private device

On Wed, Jan 08, 2025 at 09:30:26AM -0400, Jason Gunthorpe wrote:
> On Tue, Jan 07, 2025 at 10:27:15PM +0800, Xu Yilun wrote:
> > Add a flag for ioctl(VFIO_DEVICE_BIND_IOMMUFD) to mark a device as
> > for private assignment. For these private assigned devices, disallow
> > host accessing their MMIO resources.
> 
> Why? Shouldn't the VMM simply not call mmap? Why does the kernel have
> to enforce this?

MM.. maybe I should not say 'host', instead 'userspace'.

I think the kernel part VMM (KVM) has the responsibility to enforce the
correct behavior of the userspace part VMM (QEMU). QEMU has no way to
touch private memory/MMIO intentionally or accidently. IIUC that's one
of the initiative guest_memfd is introduced for private memory. Private
MMIO follows.

Thanks,
Yilun

> 
> Jason