linux-kernel - Re: [PATCH RFCv2 00/13] iommu: Add MSI mapping support with nested SMMU

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Message-ID: <Z6Psj2+ingBos0Bn@Asurada-Nvidia>
Date: Wed, 5 Feb 2025 14:56:15 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: Jacob Pan <jacob.pan@...ux.microsoft.com>
CC: <will@...nel.org>, <robin.murphy@....com>, <jgg@...dia.com>,
	<kevin.tian@...el.com>, <tglx@...utronix.de>, <maz@...nel.org>,
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Subject: Re: [PATCH RFCv2 00/13] iommu: Add MSI mapping support with nested
 SMMU

On Wed, Feb 05, 2025 at 02:49:04PM -0800, Jacob Pan wrote:
> > There have been two approaches to solve this problem:
> > 1. Create an identity mapping in the stage-1. VMM could insert a few
> > RMRs (Reserved Memory Regions) in guest's IORT. Then the guest kernel
> > would fetch these RMR entries from the IORT and create an
> > IOMMU_RESV_DIRECT region per iommu group for a direct mapping.
> > Eventually, the mappings would look like: IOVA (0x8000000) === IPA
> > (0x8000000) ===> 0x20200000 This requires an IOMMUFD ioctl for kernel
> > and VMM to agree on the IPA.
> 
> Should this RMR be in a separate range than MSI_IOVA_BASE? The guest
> will have MSI_IOVA_BASE in a reserved region already, no?
> e.g. # cat
> /sys/bus/pci/devices/0015\:01\:00.0/iommu_group/reserved_regions
> 0x0000000008000000 0x00000000080fffff msi

No. In Patch-9, the driver-defined MSI_IOVA_BASE will be ignored if
userspace has assigned IOMMU_OPTION_SW_MSI_START/SIZE, even if they
might have the same values as the MSI_IOVA_BASE window.

The idea of MSI_IOVA_BASE in this series is a kernel default that'd
be only effective when user space doesn't care to set anything.

Thanks
Nicolin