linux-kernel - Re: [PATCH v4 1/2] kvm/arm64: Remove the creation time's mapping of MMIO regions

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Message-ID: <ad39c796-2778-df26-b0c6-231e7626a747@redhat.com>
Date:   Wed, 21 Apr 2021 16:38:28 +1000
From:   Gavin Shan <gshan@...hat.com>
To:     Keqian Zhu <zhukeqian1@...wei.com>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu, Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH v4 1/2] kvm/arm64: Remove the creation time's mapping of
 MMIO regions

Hi Keqian,

On 4/16/21 12:03 AM, Keqian Zhu wrote:
> The MMIO regions may be unmapped for many reasons and can be remapped
> by stage2 fault path. Map MMIO regions at creation time becomes a
> minor optimization and makes these two mapping path hard to sync.
> 
> Remove the mapping code while keep the useful sanity check.
> 
> Signed-off-by: Keqian Zhu <zhukeqian1@...wei.com>
> ---
>   arch/arm64/kvm/mmu.c | 38 +++-----------------------------------
>   1 file changed, 3 insertions(+), 35 deletions(-)
> 

After removing the logic to create stage2 mapping for VM_PFNMAP region,
I think the "do { } while" loop becomes unnecessary and can be dropped
completely. It means the only sanity check is to see if the memory slot
overflows IPA space or not. In that case, KVM_MR_FLAGS_ONLY can be
ignored because the memory slot's base address and length aren't changed
when we have KVM_MR_FLAGS_ONLY.

It seems the patch isn't based on "next" branch because find_vma() was
replaced by find_vma_intersection() by one of my patches :)

Thanks,
Gavin

> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 8711894db8c2..c59af5ca01b0 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1301,7 +1301,6 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>   {
>   	hva_t hva = mem->userspace_addr;
>   	hva_t reg_end = hva + mem->memory_size;
> -	bool writable = !(mem->flags & KVM_MEM_READONLY);
>   	int ret = 0;
>   
>   	if (change != KVM_MR_CREATE && change != KVM_MR_MOVE &&
> @@ -1318,8 +1317,7 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>   	mmap_read_lock(current->mm);
>   	/*
>   	 * A memory region could potentially cover multiple VMAs, and any holes
> -	 * between them, so iterate over all of them to find out if we can map
> -	 * any of them right now.
> +	 * between them, so iterate over all of them.
>   	 *
>   	 *     +--------------------------------------------+
>   	 * +---------------+----------------+   +----------------+
> @@ -1330,50 +1328,20 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>   	 */
>   	do {
>   		struct vm_area_struct *vma = find_vma(current->mm, hva);
> -		hva_t vm_start, vm_end;
>   
>   		if (!vma || vma->vm_start >= reg_end)
>   			break;
>   
> -		/*
> -		 * Take the intersection of this VMA with the memory region
> -		 */
> -		vm_start = max(hva, vma->vm_start);
> -		vm_end = min(reg_end, vma->vm_end);
> -
>   		if (vma->vm_flags & VM_PFNMAP) {
> -			gpa_t gpa = mem->guest_phys_addr +
> -				    (vm_start - mem->userspace_addr);
> -			phys_addr_t pa;
> -
> -			pa = (phys_addr_t)vma->vm_pgoff << PAGE_SHIFT;
> -			pa += vm_start - vma->vm_start;
> -
>   			/* IO region dirty page logging not allowed */
>   			if (memslot->flags & KVM_MEM_LOG_DIRTY_PAGES) {
>   				ret = -EINVAL;
> -				goto out;
> -			}
> -
> -			ret = kvm_phys_addr_ioremap(kvm, gpa, pa,
> -						    vm_end - vm_start,
> -						    writable);
> -			if (ret)
>   				break;
> +			}
>   		}
> -		hva = vm_end;
> +		hva = min(reg_end, vma->vm_end);
>   	} while (hva < reg_end);
>   
> -	if (change == KVM_MR_FLAGS_ONLY)
> -		goto out;
> -
> -	spin_lock(&kvm->mmu_lock);
> -	if (ret)
> -		unmap_stage2_range(&kvm->arch.mmu, mem->guest_phys_addr, mem->memory_size);
> -	else if (!cpus_have_final_cap(ARM64_HAS_STAGE2_FWB))
> -		stage2_flush_memslot(kvm, memslot);
> -	spin_unlock(&kvm->mmu_lock);
> -out:
>   	mmap_read_unlock(current->mm);
>   	return ret;
>   }
>