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

[Bob Liu]

On 2017/12/14 11:38, Lu Baolu wrote:
> 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@xxxxxxxxx>
>>>>
>>>> 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).
> 

But who/what kind of real device has the requirement to access a kernel VA?
Looks like SVM_FLAG_SUPERVISOR_MODE is used by nobody?

Cheers,
Liubo

> 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
>