Re: [PATCH v2 3/3] KVM: arm64: Mark the page dirty only if the fault is handled successfully

[wangyanan (Y)]

On 2021/1/13 23:51, Will Deacon wrote:

On Wed, Dec 16, 2020 at 08:28:44PM +0800, Yanan Wang wrote:

We now mark the page dirty and set the bitmap before calling fault handlers
in user_mem_abort(), and we might end up having spurious dirty pages if
update of permissions or mapping has failed.
So, mark the page dirty only if the fault is handled successfully.

Let the guest directly enter again but not return to userspace if we were
trying to recreate the same mapping or only change access permissions
with BBM, which is not permitted in the mapping path.

Signed-off-by: Yanan Wang 
---
  arch/arm64/kvm/mmu.c | 18 ++
  1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 75814a02d189..72e516a10914 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -879,11 +879,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, 
phys_addr_t fault_ipa,
if (vma_pagesize == PAGE_SIZE && !force_pte)
vma_pagesize = transparent_hugepage_adjust(memslot, hva,
   , _ipa);
-   if (writable) {
+   if (writable)
prot |= KVM_PGTABLE_PROT_W;
-   kvm_set_pfn_dirty(pfn);
-   mark_page_dirty(kvm, gfn);
-   }
  
  	if (fault_status != FSC_PERM && !device)

clean_dcache_guest_page(pfn, vma_pagesize);
@@ -911,6 +908,19 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, 
phys_addr_t fault_ipa,
 memcache);
}
  
+	/* Mark the page dirty only if the fault is handled successfully */

+   if (writable && !ret) {
+   kvm_set_pfn_dirty(pfn);
+   mark_page_dirty(kvm, gfn);
+   }
+
+   /* Let the guest directly enter again if we were trying to recreate the
+* same mapping or only change access permissions with BBM, which is not
+* permitted in the mapping path.
+*/
+   if (ret == -EAGAIN)
+   ret = 0;

Maybe just 'return ret != -EAGAIN ? ret : 0;' at the end of the function?

Yes, it's more concise.

Re: [PATCH v2 3/3] KVM: arm64: Mark the page dirty only if the fault is handled successfully

[Will Deacon]

On Wed, Dec 16, 2020 at 08:28:44PM +0800, Yanan Wang wrote:
> We now mark the page dirty and set the bitmap before calling fault handlers
> in user_mem_abort(), and we might end up having spurious dirty pages if
> update of permissions or mapping has failed.
> So, mark the page dirty only if the fault is handled successfully.
> 
> Let the guest directly enter again but not return to userspace if we were
> trying to recreate the same mapping or only change access permissions
> with BBM, which is not permitted in the mapping path.
> 
> Signed-off-by: Yanan Wang 
> ---
>  arch/arm64/kvm/mmu.c | 18 ++
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 75814a02d189..72e516a10914 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -879,11 +879,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, 
> phys_addr_t fault_ipa,
>   if (vma_pagesize == PAGE_SIZE && !force_pte)
>   vma_pagesize = transparent_hugepage_adjust(memslot, hva,
>  , _ipa);
> - if (writable) {
> + if (writable)
>   prot |= KVM_PGTABLE_PROT_W;
> - kvm_set_pfn_dirty(pfn);
> - mark_page_dirty(kvm, gfn);
> - }
>  
>   if (fault_status != FSC_PERM && !device)
>   clean_dcache_guest_page(pfn, vma_pagesize);
> @@ -911,6 +908,19 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, 
> phys_addr_t fault_ipa,
>memcache);
>   }
>  
> + /* Mark the page dirty only if the fault is handled successfully */
> + if (writable && !ret) {
> + kvm_set_pfn_dirty(pfn);
> + mark_page_dirty(kvm, gfn);
> + }
> +
> + /* Let the guest directly enter again if we were trying to recreate the
> +  * same mapping or only change access permissions with BBM, which is not
> +  * permitted in the mapping path.
> +  */
> + if (ret == -EAGAIN)
> + ret = 0;

Maybe just 'return ret != -EAGAIN ? ret : 0;' at the end of the function?

Will

[PATCH v2 3/3] KVM: arm64: Mark the page dirty only if the fault is handled successfully

[Yanan Wang]

We now mark the page dirty and set the bitmap before calling fault handlers
in user_mem_abort(), and we might end up having spurious dirty pages if
update of permissions or mapping has failed.
So, mark the page dirty only if the fault is handled successfully.

Let the guest directly enter again but not return to userspace if we were
trying to recreate the same mapping or only change access permissions
with BBM, which is not permitted in the mapping path.

Signed-off-by: Yanan Wang 
---
 arch/arm64/kvm/mmu.c | 18 ++
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 75814a02d189..72e516a10914 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -879,11 +879,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, 
phys_addr_t fault_ipa,
if (vma_pagesize == PAGE_SIZE && !force_pte)
vma_pagesize = transparent_hugepage_adjust(memslot, hva,
   , _ipa);
-   if (writable) {
+   if (writable)
prot |= KVM_PGTABLE_PROT_W;
-   kvm_set_pfn_dirty(pfn);
-   mark_page_dirty(kvm, gfn);
-   }
 
if (fault_status != FSC_PERM && !device)
clean_dcache_guest_page(pfn, vma_pagesize);
@@ -911,6 +908,19 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, 
phys_addr_t fault_ipa,
 memcache);
}
 
+   /* Mark the page dirty only if the fault is handled successfully */
+   if (writable && !ret) {
+   kvm_set_pfn_dirty(pfn);
+   mark_page_dirty(kvm, gfn);
+   }
+
+   /* Let the guest directly enter again if we were trying to recreate the
+* same mapping or only change access permissions with BBM, which is not
+* permitted in the mapping path.
+*/
+   if (ret == -EAGAIN)
+   ret = 0;
+
 out_unlock:
spin_unlock(>mmu_lock);
kvm_set_pfn_accessed(pfn);
-- 
2.19.1