[PATCH] kvm: Map PFN-type memory regions as writable (if possible)

[PATCH] kvm: Map PFN-type memory regions as writable (if possible)

[KarimAllah Ahmed]

For EPT-violations that are triggered by a read, the pages are also mapped with
write permissions (if their memory region is also writable). That would avoid
getting yet another fault on the same page when a write occurs.

This optimization only happens when you have a "struct page" backing the memory
region. So also enable it for memory regions that do not have a "struct page".

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: kvm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
---
 virt/kvm/kvm_main.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 97da45e..0efb089 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1534,6 +1534,8 @@ static kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
 			goto retry;
 		if (r < 0)
 			pfn = KVM_PFN_ERR_FAULT;
+		if (writable)
+			*writable = true;
 	} else {
 		if (async && vma_is_valid(vma, write_fault))
 			*async = true;
-- 
2.7.4

Re: [PATCH] kvm: Map PFN-type memory regions as writable (if possible)

[Paolo Bonzini]

On 17/01/2018 18:26, KarimAllah Ahmed wrote:
> For EPT-violations that are triggered by a read, the pages are also mapped with
> write permissions (if their memory region is also writable). That would avoid
> getting yet another fault on the same page when a write occurs.
> 
> This optimization only happens when you have a "struct page" backing the memory
> region. So also enable it for memory regions that do not have a "struct page".
> 
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: kvm@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
> ---
>  virt/kvm/kvm_main.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 97da45e..0efb089 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1534,6 +1534,8 @@ static kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
>  			goto retry;
>  		if (r < 0)
>  			pfn = KVM_PFN_ERR_FAULT;
> +		if (writable)
> +			*writable = true;
>  	} else {
>  		if (async && vma_is_valid(vma, write_fault))
>  			*async = true;
> 

Looks good, but it should be in hva_to_pfn_remapped.

Paolo

[PATCH v2] kvm: Map PFN-type memory regions as writable (if possible)

[KarimAllah Ahmed]

For EPT-violations that are triggered by a read, the pages are also mapped with
write permissions (if their memory region is also writable). That would avoid
getting yet another fault on the same page when a write occurs.

This optimization only happens when you have a "struct page" backing the memory
region. So also enable it for memory regions that do not have a "struct page".

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: kvm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>

---
v2:
- Move setting writable to hva_to_pfn_remapped
- Extend hva_to_pfn_remapped interface to accept writable as a parameter
---
 virt/kvm/kvm_main.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 97da45e..88702d5 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1438,7 +1438,8 @@ static bool vma_is_valid(struct vm_area_struct *vma, bool write_fault)
 
 static int hva_to_pfn_remapped(struct vm_area_struct *vma,
 			       unsigned long addr, bool *async,
-			       bool write_fault, kvm_pfn_t *p_pfn)
+			       bool write_fault, bool *writable,
+			       kvm_pfn_t *p_pfn)
 {
 	unsigned long pfn;
 	int r;
@@ -1464,6 +1465,8 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
 
 	}
 
+	if (writable)
+		*writable = true;
 
 	/*
 	 * Get a reference here because callers of *hva_to_pfn* and
@@ -1529,7 +1532,7 @@ static kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
 	if (vma == NULL)
 		pfn = KVM_PFN_ERR_FAULT;
 	else if (vma->vm_flags & (VM_IO | VM_PFNMAP)) {
-		r = hva_to_pfn_remapped(vma, addr, async, write_fault, &pfn);
+		r = hva_to_pfn_remapped(vma, addr, async, write_fault, writable, &pfn);
 		if (r == -EAGAIN)
 			goto retry;
 		if (r < 0)
-- 
2.7.4

Re: [PATCH v2] kvm: Map PFN-type memory regions as writable (if possible)

[Paolo Bonzini]

On 17/01/2018 19:18, KarimAllah Ahmed wrote:
> For EPT-violations that are triggered by a read, the pages are also mapped with
> write permissions (if their memory region is also writable). That would avoid
> getting yet another fault on the same page when a write occurs.
> 
> This optimization only happens when you have a "struct page" backing the memory
> region. So also enable it for memory regions that do not have a "struct page".
> 
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: kvm@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
> 
> ---
> v2:
> - Move setting writable to hva_to_pfn_remapped
> - Extend hva_to_pfn_remapped interface to accept writable as a parameter

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

> ---
>  virt/kvm/kvm_main.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 97da45e..88702d5 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1438,7 +1438,8 @@ static bool vma_is_valid(struct vm_area_struct *vma, bool write_fault)
>  
>  static int hva_to_pfn_remapped(struct vm_area_struct *vma,
>  			       unsigned long addr, bool *async,
> -			       bool write_fault, kvm_pfn_t *p_pfn)
> +			       bool write_fault, bool *writable,
> +			       kvm_pfn_t *p_pfn)
>  {
>  	unsigned long pfn;
>  	int r;
> @@ -1464,6 +1465,8 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
>  
>  	}
>  
> +	if (writable)
> +		*writable = true;
>  
>  	/*
>  	 * Get a reference here because callers of *hva_to_pfn* and
> @@ -1529,7 +1532,7 @@ static kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
>  	if (vma == NULL)
>  		pfn = KVM_PFN_ERR_FAULT;
>  	else if (vma->vm_flags & (VM_IO | VM_PFNMAP)) {
> -		r = hva_to_pfn_remapped(vma, addr, async, write_fault, &pfn);
> +		r = hva_to_pfn_remapped(vma, addr, async, write_fault, writable, &pfn);
>  		if (r == -EAGAIN)
>  			goto retry;
>  		if (r < 0)
>