linux-kernel - Re: [RFC PATCH v2 04/10] iommu/riscv: add iotlb_sync

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Date: Mon, 17 Jun 2024 21:43:35 +0800
From: Zong Li <zong.li@...ive.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>
Cc: joro@...tes.org, will@...nel.org, robin.murphy@....com, 
	tjeznach@...osinc.com, paul.walmsley@...ive.com, palmer@...belt.com, 
	aou@...s.berkeley.edu, jgg@...pe.ca, kevin.tian@...el.com, 
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev, 
	linux-riscv@...ts.infradead.org
Subject: Re: [RFC PATCH v2 04/10] iommu/riscv: add iotlb_sync_map operation support

On Sat, Jun 15, 2024 at 11:17 AM Baolu Lu <baolu.lu@...ux.intel.com> wrote:
>
> On 6/14/24 10:21 PM, Zong Li wrote:
> > Add iotlb_sync_map operation for flush IOTLB. Software must
> > flush the IOTLB after each page table.
> >
> > Signed-off-by: Zong Li<zong.li@...ive.com>
> > ---
> >   drivers/iommu/riscv/Makefile |  1 +
> >   drivers/iommu/riscv/iommu.c  | 11 +++++++++++
> >   2 files changed, 12 insertions(+)
> >
> > diff --git a/drivers/iommu/riscv/Makefile b/drivers/iommu/riscv/Makefile
> > index d36625a1fd08..f02ce6ebfbd0 100644
> > --- a/drivers/iommu/riscv/Makefile
> > +++ b/drivers/iommu/riscv/Makefile
> > @@ -1,3 +1,4 @@
> >   # SPDX-License-Identifier: GPL-2.0-only
> >   obj-$(CONFIG_RISCV_IOMMU) += iommu.o iommu-platform.o iommu-pmu.o
> >   obj-$(CONFIG_RISCV_IOMMU_PCI) += iommu-pci.o
> > +obj-$(CONFIG_SIFIVE_IOMMU) += iommu-sifive.o
> > diff --git a/drivers/iommu/riscv/iommu.c b/drivers/iommu/riscv/iommu.c
> > index 9aeb4b20c145..df7aeb2571ae 100644
> > --- a/drivers/iommu/riscv/iommu.c
> > +++ b/drivers/iommu/riscv/iommu.c
> > @@ -1115,6 +1115,16 @@ static void riscv_iommu_iotlb_sync(struct iommu_domain *iommu_domain,
> >       riscv_iommu_iotlb_inval(domain, gather->start, gather->end);
> >   }
> >
> > +static int riscv_iommu_iotlb_sync_map(struct iommu_domain *iommu_domain,
> > +                                   unsigned long iova, size_t size)
> > +{
> > +     struct riscv_iommu_domain *domain = iommu_domain_to_riscv(iommu_domain);
> > +
> > +     riscv_iommu_iotlb_inval(domain, iova, iova + size - 1);
>
> Does the RISC-V IOMMU architecture always cache the non-present or
> erroneous translation entries? If so, can you please provide more
> context in the commit message?
>
> If not, why do you want to flush the cache when building a new
> translation?
>

It seems to me that we can indeed remove this operation, because it
may be too aggressive given the following situation.

I added it for updating the MSI mapping when we change the irq
affinity of a pass-through device to another vCPU. The RISC-V IOMMU
spec allows MSI translation to go through the MSI flat table, MRIF, or
the normal page table. In the case of the normal page table, the MSI
mapping is created in the second-stage page table, mapping the GPA of
the guest's supervisor interrupt file to the HPA of host's guest
interrupt file. This MSI mapping needs to be updated when the HPA of
host's guest interrupt file is changed.

I think we can invalidate the cache after updating the MSI mapping,
rather than adding the iotlb_sync_map() operation for every mapping
created. Does it also make sense to you? If so, I will remove it in
the next version. Thanks.

> Best regards,
> baolu