Re: [PATCH v3 2/2] kvm/arm64: Try stage2 block mapping for host device MMIO

[Marc Zyngier <maz@kernel.org>]

+ Santosh, who found some interesting bugs in that area before.

On Wed, 14 Apr 2021 07:51:09 +0100,
Keqian Zhu <zhukeqian1@huawei.com> 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@huawei.com>
> ---
>  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@nvidia.com/

-- 
Without deviation from the norm, progress is not possible.