Re: [PATCH v2 2/4] vfio: Move the Intel no-snoop control off of IOMMU_CACHE

Alex Williamson
2022-04-08T15:48:07Z

On Thu, 7 Apr 2022 12:23:45 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> IOMMU_CACHE means "normal DMA to this iommu_domain's IOVA should be cache
> coherent" and is used by the DMA API. The definition allows for special
> non-coherent DMA to exist - ie processing of the no-snoop flag in PCIe
> TLPs - so long as this behavior is opt-in by the device driver.
>
> The flag is mainly used by the DMA API to synchronize the IOMMU setting
> with the expected cache behavior of the DMA master. eg based on
> dev_is_dma_coherent() in some case.
>
> For Intel IOMMU IOMMU_CACHE was redefined to mean 'force all DMA to be
> cache coherent' which has the practical effect of causing the IOMMU to
> ignore the no-snoop bit in a PCIe TLP.
>
> x86 platforms are always IOMMU_CACHE, so Intel should ignore this flag.
>
> Instead use the new domain op enforce_cache_coherency() which causes every
> IOPTE created in the domain to have the no-snoop blocking behavior.
>
> Reconfigure VFIO to always use IOMMU_CACHE and call
> enforce_cache_coherency() to operate the special Intel behavior.
>
> Remove the IOMMU_CACHE test from Intel IOMMU.
>
> Ultimately VFIO plumbs the result of enforce_cache_coherency() back into
> the x86 platform code through kvm_arch_register_noncoherent_dma() which
> controls if the WBINVD instruction is available in the guest. No other
> arch implements kvm_arch_register_noncoherent_dma().

I think this last sentence is alluding to it, but I wish the user
visible change to VFIO_DMA_CC_IOMMU on non-x86 were more explicit.
Perhaps for the last sentence:

No other archs implement kvm_arch_register_noncoherent_dma() nor are
there any other known consumers of VFIO_DMA_CC_IOMMU that might be
affected by the user visible result change on non-x86 archs.

Otherwise,

Acked-by: Alex Williamson <alex.williamson@redhat.com>

>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
> drivers/iommu/intel/iommu.c | 7 ++-----
> drivers/vfio/vfio_iommu_type1.c | 30 +++++++++++++++++++-----------
> include/linux/intel-iommu.h | 1 -
> 3 files changed, 21 insertions(+), 17 deletions(-)
>
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index f08611a6cc4799..8f3674e997df06 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -641,7 +641,6 @@ static unsigned long domain_super_pgsize_bitmap(struct dmar_domain *domain)
> static void domain_update_iommu_cap(struct dmar_domain *domain)
> {
> domain_update_iommu_coherency(domain);
> - domain->iommu_snooping = domain_update_iommu_snooping(NULL);
> domain->iommu_superpage = domain_update_iommu_superpage(domain, NULL);
>
> /*
> @@ -4283,7 +4282,6 @@ static int md_domain_init(struct dmar_domain *domain, int guest_width)
> domain->agaw = width_to_agaw(adjust_width);
>
> domain->iommu_coherency = false;
> - domain->iommu_snooping = false;
> domain->iommu_superpage = 0;
> domain->max_addr = 0;
>
> @@ -4422,8 +4420,7 @@ static int intel_iommu_map(struct iommu_domain *domain,
> prot |= DMA_PTE_READ;
> if (iommu_prot & IOMMU_WRITE)
> prot |= DMA_PTE_WRITE;
> - if (((iommu_prot & IOMMU_CACHE) && dmar_domain->iommu_snooping) ||
> - dmar_domain->enforce_no_snoop)
> + if (dmar_domain->enforce_no_snoop)
> prot |= DMA_PTE_SNP;
>
> max_addr = iova + size;
> @@ -4550,7 +4547,7 @@ static bool intel_iommu_enforce_cache_coherency(struct iommu_domain *domain)
> {
> struct dmar_domain *dmar_domain = to_dmar_domain(domain);
>
> - if (!dmar_domain->iommu_snooping)
> + if (!domain_update_iommu_snooping(NULL))
> return false;
> dmar_domain->enforce_no_snoop = true;
> return true;
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 9394aa9444c10c..c13b9290e35759 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -84,8 +84,8 @@ struct vfio_domain {
> struct iommu_domain *domain;
> struct list_head next;
> struct list_head group_list;
> - int prot; /* IOMMU_CACHE */
> - bool fgsp; /* Fine-grained super pages */
> + bool fgsp : 1; /* Fine-grained super pages */
> + bool enforce_cache_coherency : 1;
> };
>
> struct vfio_dma {
> @@ -1461,7 +1461,7 @@ static int vfio_iommu_map(struct vfio_iommu *iommu, dma_addr_t iova,
>
> list_for_each_entry(d, &iommu->domain_list, next) {
> ret = iommu_map(d->domain, iova, (phys_addr_t)pfn << PAGE_SHIFT,
> - npage << PAGE_SHIFT, prot | d->prot);
> + npage << PAGE_SHIFT, prot | IOMMU_CACHE);
> if (ret)
> goto unwind;
>
> @@ -1771,7 +1771,7 @@ static int vfio_iommu_replay(struct vfio_iommu *iommu,
> }
>
> ret = iommu_map(domain->domain, iova, phys,
> - size, dma->prot | domain->prot);
> + size, dma->prot | IOMMU_CACHE);
> if (ret) {
> if (!dma->iommu_mapped) {
> vfio_unpin_pages_remote(dma, iova,
> @@ -1859,7 +1859,7 @@ static void vfio_test_domain_fgsp(struct vfio_domain *domain)
> return;
>
> ret = iommu_map(domain->domain, 0, page_to_phys(pages), PAGE_SIZE * 2,
> - IOMMU_READ | IOMMU_WRITE | domain->prot);
> + IOMMU_READ | IOMMU_WRITE | IOMMU_CACHE);
> if (!ret) {
> size_t unmapped = iommu_unmap(domain->domain, 0, PAGE_SIZE);
>
> @@ -2267,8 +2267,15 @@ static int vfio_iommu_type1_attach_group(void *iommu_data,
> goto out_detach;
> }
>
> - if (iommu_capable(bus, IOMMU_CAP_CACHE_COHERENCY))
> - domain->prot |= IOMMU_CACHE;
> + /*
> + * If the IOMMU can block non-coherent operations (ie PCIe TLPs with
> + * no-snoop set) then VFIO always turns this feature on because on Intel
> + * platforms it optimizes KVM to disable wbinvd emulation.
> + */
> + if (domain->domain->ops->enforce_cache_coherency)
> + domain->enforce_cache_coherency =
> + domain->domain->ops->enforce_cache_coherency(
> + domain->domain);
>
> /*
> * Try to match an existing compatible domain. We don't want to
> @@ -2279,7 +2286,8 @@ static int vfio_iommu_type1_attach_group(void *iommu_data,
> */
> list_for_each_entry(d, &iommu->domain_list, next) {
> if (d->domain->ops == domain->domain->ops &&
> - d->prot == domain->prot) {
> + d->enforce_cache_coherency ==
> + domain->enforce_cache_coherency) {
> iommu_detach_group(domain->domain, group->iommu_group);
> if (!iommu_attach_group(d->domain,
> group->iommu_group)) {
> @@ -2611,14 +2619,14 @@ static void vfio_iommu_type1_release(void *iommu_data)
> kfree(iommu);
> }
>
> -static int vfio_domains_have_iommu_cache(struct vfio_iommu *iommu)
> +static int vfio_domains_have_enforce_cache_coherency(struct vfio_iommu *iommu)
> {
> struct vfio_domain *domain;
> int ret = 1;
>
> mutex_lock(&iommu->lock);
> list_for_each_entry(domain, &iommu->domain_list, next) {
> - if (!(domain->prot & IOMMU_CACHE)) {
> + if (!(domain->enforce_cache_coherency)) {
> ret = 0;
> break;
> }
> @@ -2641,7 +2649,7 @@ static int vfio_iommu_type1_check_extension(struct vfio_iommu *iommu,
> case VFIO_DMA_CC_IOMMU:
> if (!iommu)
> return 0;
> - return vfio_domains_have_iommu_cache(iommu);
> + return vfio_domains_have_enforce_cache_coherency(iommu);
> default:
> return 0;
> }
> diff --git a/include/linux/intel-iommu.h b/include/linux/intel-iommu.h
> index 1f930c0c225d94..bc39f633efdf03 100644
> --- a/include/linux/intel-iommu.h
> +++ b/include/linux/intel-iommu.h
> @@ -539,7 +539,6 @@ struct dmar_domain {
>
> u8 has_iotlb_device: 1;
> u8 iommu_coherency: 1; /* indicate coherency of iommu access */
> - u8 iommu_snooping: 1; /* indicate snooping control feature */
> u8 enforce_no_snoop : 1; /* Create IOPTEs with snoop control */
>
> struct list_head devices; /* all devices' list */