linux-kernel - Re: [PATCH v3 16/23] Documentation: userspace-api: iommufd: Update vQUEUE

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Message-ID: <aBRYVkOKfFGMb5Y+@Asurada-Nvidia>
Date: Thu, 1 May 2025 22:29:58 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Bagas Sanjaya <bagasdotme@...il.com>
CC: <jgg@...dia.com>, <kevin.tian@...el.com>, <corbet@....net>,
	<will@...nel.org>, <robin.murphy@....com>, <joro@...tes.org>,
	<thierry.reding@...il.com>, <vdumpa@...dia.com>, <jonathanh@...dia.com>,
	<shuah@...nel.org>, <jsnitsel@...hat.com>, <nathan@...nel.org>,
	<peterz@...radead.org>, <yi.l.liu@...el.com>, <mshavit@...gle.com>,
	<praan@...gle.com>, <zhangzekun11@...wei.com>, <iommu@...ts.linux.dev>,
	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-tegra@...r.kernel.org>,
	<linux-kselftest@...r.kernel.org>, <patches@...ts.linux.dev>,
	<mochs@...dia.com>, <alok.a.tiwari@...cle.com>, <vasant.hegde@....com>
Subject: Re: [PATCH v3 16/23] Documentation: userspace-api: iommufd: Update
 vQUEUE

On Fri, May 02, 2025 at 10:50:07AM +0700, Bagas Sanjaya wrote:
> On Thu, May 01, 2025 at 04:01:22PM -0700, Nicolin Chen wrote:
> > +- IOMMUFD_OBJ_VQUEUE, representing a hardware accelerated virtual queue, as a
> > +  subset of IOMMU's virtualization features, for the IOMMU HW to directly read
> > +  or write the virtual queue memory owned by a guest OS. This HW-acceleration
> > +  allows VM to work with the IOMMU HW directly without a VM Exit, i.e. reducing
> > +  overhead from the hypercalls. Along with this vQUEUE object, iommufd provides
> > +  user space an mmap interface for VMM to mmap a physical MMIO region from the
> > +  host physical address space to the guest physical address space, allowing the
> > +  guest OS to control the allocated vQUEUE HW. Thus, when allocating a vQUEUE,
> > +  the VMM must request a pair of VMA info (vm_pgoff/size) for an mmap syscall.
> > +  The length argument of an mmap syscall can be smaller than the given size for
> > +  a partial mmap, but the addr argument of the mmap syscall should never offset
> > +  from the returned vm_pgoff, which implies that an mmap will always start from
> 
> Did you mean never be offset from returned vm_pgoff?

Yes. Will fix this.

> > +  the beginning of the physical MMIO region.
> > +
> 
> Confused...

Meaning that VMM should just use the given vm_pgoff as is, without
adding any offset to the vm_pgoff.

Thanks
Nicolin