Re: [PATCH 1/2] mm: Add kernel MMU notifier to manage IOTLB/DEVTLB

[Lu Baolu <baolu.lu@linux.intel.com>]

Hi,

On 12/14/2017 11:10 AM, Bob Liu wrote:
> On 2017/12/14 9:02, Lu Baolu wrote:
>> > From: Huang Ying <ying.huang@intel.com>
>> > 
>> > Shared Virtual Memory (SVM) allows a kernel memory mapping to be
>> > shared between CPU and and a device which requested a supervisor
>> > PASID. Both devices and IOMMU units have TLBs that cache entries
>> > from CPU's page tables. We need to get a chance to flush them at
>> > the same time when we flush the CPU TLBs.
>> > 
>> > We already have an existing MMU notifiers for userspace updates,
>> > however we lack the same thing for kernel page table updates. To
> Sorry, I didn't get which situation need this notification.
> Could you please describe the full scenario?

Okay.

1. When an SVM capable driver calls intel_svm_bind_mm() with
    SVM_FLAG_SUPERVISOR_MODE set in the @flags, the kernel
    memory page mappings will be shared between CPUs and
    the DMA remapping agent (a.k.a. IOMMU). The page table
    entries will also be cached in both IOTLB (located in IOMMU)
    and the DEVTLB (located in device).

2. When vmalloc/vfree interfaces are called, the page mappings
    for kernel memory might get changed. And current code calls
    flush_tlb_kernel_range() to flush CPU TLBs only. The IOTLB or
    DevTLB will be stale compared to that on the cpu for kernel
    mappings.

We need a kernel mmu notification to flush TLBs in IOMMU and
devices as well.

Best regards,
Lu Baolu