+ Santosh, who found some interesting bugs in that area before.
On Wed, 14 Apr 2021 07:51:09 +0100,
Keqian Zhu <zhukeqian1@xxxxxxxxxx> wrote:
>
> The MMIO region of a device maybe huge (GB level), try to use
> block mapping in stage2 to speedup both map and unmap.
>
> Compared to normal memory mapping, we should consider two more
> points when try block mapping for MMIO region:
>
> 1. For normal memory mapping, the PA(host physical address) and
> HVA have same alignment within PUD_SIZE or PMD_SIZE when we use
> the HVA to request hugepage, so we don't need to consider PA
> alignment when verifing block mapping. But for device memory
> mapping, the PA and HVA may have different alignment.
>
> 2. For normal memory mapping, we are sure hugepage size properly
> fit into vma, so we don't check whether the mapping size exceeds
> the boundary of vma. But for device memory mapping, we should pay
> attention to this.
>
> This adds device_rough_page_shift() to check these two points when
> selecting block mapping size.
>
> Signed-off-by: Keqian Zhu <zhukeqian1@xxxxxxxxxx>
> ---
> arch/arm64/kvm/mmu.c | 37 +++++++++++++++++++++++++++++++++----
> 1 file changed, 33 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index c59af5ca01b0..1a6d96169d60 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -624,6 +624,31 @@ static void kvm_send_hwpoison_signal(unsigned long address, short lsb)
> send_sig_mceerr(BUS_MCEERR_AR, (void __user *)address, lsb, current);
> }
>
> +/*
> + * Find a max mapping size that properly insides the vma. And hva and pa must
> + * have the same alignment to this mapping size. It's rough as there are still
> + * other restrictions, will be checked by fault_supports_stage2_huge_mapping().
> + */
> +static short device_rough_page_shift(struct vm_area_struct *vma,
> + unsigned long hva)
My earlier question still stands. Under which circumstances would this
function return something that is *not* the final mapping size? I
really don't see a reason why this would not return the final mapping
size.
> +{
> + phys_addr_t pa = (vma->vm_pgoff << PAGE_SHIFT) + (hva - vma->vm_start);
> +
> +#ifndef __PAGETABLE_PMD_FOLDED
> + if ((hva & (PUD_SIZE - 1)) == (pa & (PUD_SIZE - 1)) &&
> + ALIGN_DOWN(hva, PUD_SIZE) >= vma->vm_start &&
> + ALIGN(hva, PUD_SIZE) <= vma->vm_end)
> + return PUD_SHIFT;
> +#endif
> +
> + if ((hva & (PMD_SIZE - 1)) == (pa & (PMD_SIZE - 1)) &&
> + ALIGN_DOWN(hva, PMD_SIZE) >= vma->vm_start &&
> + ALIGN(hva, PMD_SIZE) <= vma->vm_end)
> + return PMD_SHIFT;
> +
> + return PAGE_SHIFT;
> +}
> +
> static bool fault_supports_stage2_huge_mapping(struct kvm_memory_slot *memslot,
> unsigned long hva,
> unsigned long map_size)
> @@ -769,7 +794,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> return -EFAULT;
> }
>
> - /* Let's check if we will get back a huge page backed by hugetlbfs */
> + /*
> + * Let's check if we will get back a huge page backed by hugetlbfs, or
> + * get block mapping for device MMIO region.
> + */
> mmap_read_lock(current->mm);
> vma = find_vma_intersection(current->mm, hva, hva + 1);
> if (unlikely(!vma)) {
> @@ -780,11 +808,12 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>
> if (is_vm_hugetlb_page(vma))
> vma_shift = huge_page_shift(hstate_vma(vma));
> + else if (vma->vm_flags & VM_PFNMAP)
> + vma_shift = device_rough_page_shift(vma, hva);
What prevents a VMA from having both VM_HUGETLB and VM_PFNMAP? This is
pretty unlikely, but I'd like to see this case catered for.
> else
> vma_shift = PAGE_SHIFT;
>
> - if (logging_active ||
> - (vma->vm_flags & VM_PFNMAP)) {
> + if (logging_active) {
> force_pte = true;
> vma_shift = PAGE_SHIFT;
> }
> @@ -855,7 +884,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>
> if (kvm_is_device_pfn(pfn)) {
> device = true;
> - force_pte = true;
> + force_pte = (vma_pagesize == PAGE_SIZE);
Why do we need to set force_pte if we are already dealing with
PAGE_SIZE? I guess you are doing this for the sake of avoiding the
call to transparent_hugepage_adjust(), right?
I'd rather you simply don't try to upgrade a device mapping by
explicitly checking for this and keep force_pte for *memory*
exclusively.
Santosh, can you please take a look at this series and try to see if
the problem you fixed in [1] (which ended up as commit 91a2c34b7d6f)
is still OK with this series?
> } else if (logging_active && !write_fault) {
> /*
> * Only actually map the page as writable if this was a write
Thanks,
M.
[1] https://lore.kernel.org/kvmarm/1603711447-11998-1-git-send-email-sashukla@xxxxxxxxxx/
--
Without deviation from the norm, progress is not possible.
