[PATCH v2] KVM: SEV: drop FOLL_WRITE for encrypted region registration

[PATCH v2] KVM: SEV: drop FOLL_WRITE for encrypted region registration

[Pankaj Gupta]

Commit 7e066cb9b71a ("KVM: SEV: Use long-term pin when registering
encrypted memory regions") added FOLL_LONGTERM to
sev_mem_enc_register_region() so anonymous guest RAM is migrated out of
MIGRATE_CMA/ZONE_MOVABLE before a long term pin. It also kept
FOLL_WRITE on the pin.

Combining FOLL_WRITE with FOLL_LONGTERM breaks registration of file-backed
guest memory, such as virtio-pmem host memory-backend-file mappings
(MAP_SHARED). GUP rejects long-term writable pins on dirty tracked file
mappings since:

commit 8ac268436e6d ("mm/gup: disallow FOLL_LONGTERM GUP-nonfast writing to file-backed mappings")
commit a6e79df92e4a ("mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings").

Region registration only requires long-term pin to prevent page migration and
does not write through this GUP pin.

Drop FOLL_WRITE and pin guest memory only with FOLL_LONGTERM.

Fixes: 7e066cb9b71a ("KVM: SEV: Use long-term pin when registering encrypted memory regions")
Cc: stable@vger.kernel.org
Suggested-by: "David Hildenbrand (Arm)" <david@kernel.org>
Link: https://lore.kernel.org/all/ad784f05-b36c-4e91-9f17-4c5b826735d0@kernel.org/
Signed-off-by: Pankaj Gupta <pankaj.gupta@amd.com>
---
v1 -> v2
- Remove FOLL_WRITE when the pin is not used for host writes

v1: https://lore.kernel.org/all/20260701144543.39582-1-pankaj.gupta@amd.com/

 arch/x86/kvm/svm/sev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 427229347876..5f2998761462 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2752,7 +2752,7 @@ int sev_mem_enc_register_region(struct kvm *kvm,
 		return -ENOMEM;
 
 	region->pages = sev_pin_memory(kvm, range->addr, range->size, &region->npages,
-				       FOLL_WRITE | FOLL_LONGTERM);
+				       FOLL_LONGTERM);
 	if (IS_ERR(region->pages)) {
 		ret = PTR_ERR(region->pages);
 		goto e_free;
-- 
2.34.1

Re: [PATCH v2] KVM: SEV: drop FOLL_WRITE for encrypted region registration

[Lorenzo Stoakes (ARM)]

On Wed, Jul 15, 2026 at 01:36:26AM -0500, Pankaj Gupta wrote:
> Commit 7e066cb9b71a ("KVM: SEV: Use long-term pin when registering
> encrypted memory regions") added FOLL_LONGTERM to
> sev_mem_enc_register_region() so anonymous guest RAM is migrated out of
> MIGRATE_CMA/ZONE_MOVABLE before a long term pin. It also kept
> FOLL_WRITE on the pin.
>
> Combining FOLL_WRITE with FOLL_LONGTERM breaks registration of file-backed
> guest memory, such as virtio-pmem host memory-backend-file mappings
> (MAP_SHARED). GUP rejects long-term writable pins on dirty tracked file
> mappings since:
>
> commit 8ac268436e6d ("mm/gup: disallow FOLL_LONGTERM GUP-nonfast writing to file-backed mappings")
> commit a6e79df92e4a ("mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings").
>
> Region registration only requires long-term pin to prevent page migration and
> does not write through this GUP pin.
>
> Drop FOLL_WRITE and pin guest memory only with FOLL_LONGTERM.
>
> Fixes: 7e066cb9b71a ("KVM: SEV: Use long-term pin when registering encrypted memory regions")
> Cc: stable@vger.kernel.org
> Suggested-by: "David Hildenbrand (Arm)" <david@kernel.org>
> Link: https://lore.kernel.org/all/ad784f05-b36c-4e91-9f17-4c5b826735d0@kernel.org/
> Signed-off-by: Pankaj Gupta <pankaj.gupta@amd.com>

Nice and simple, works for me :) So:

Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
> v1 -> v2
> - Remove FOLL_WRITE when the pin is not used for host writes
>
> v1: https://lore.kernel.org/all/20260701144543.39582-1-pankaj.gupta@amd.com/
>
>  arch/x86/kvm/svm/sev.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 427229347876..5f2998761462 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2752,7 +2752,7 @@ int sev_mem_enc_register_region(struct kvm *kvm,
>  		return -ENOMEM;
>
>  	region->pages = sev_pin_memory(kvm, range->addr, range->size, &region->npages,
> -				       FOLL_WRITE | FOLL_LONGTERM);
> +				       FOLL_LONGTERM);
>  	if (IS_ERR(region->pages)) {
>  		ret = PTR_ERR(region->pages);
>  		goto e_free;
> --
> 2.34.1
>

Cheers, Lorenzo

Re: [PATCH v2] KVM: SEV: drop FOLL_WRITE for encrypted region registration

[David Hildenbrand (Arm)]

On 7/15/26 08:36, Pankaj Gupta wrote:
> Commit 7e066cb9b71a ("KVM: SEV: Use long-term pin when registering
> encrypted memory regions") added FOLL_LONGTERM to
> sev_mem_enc_register_region() so anonymous guest RAM is migrated out of
> MIGRATE_CMA/ZONE_MOVABLE before a long term pin. It also kept
> FOLL_WRITE on the pin.
> 
> Combining FOLL_WRITE with FOLL_LONGTERM breaks registration of file-backed
> guest memory, such as virtio-pmem host memory-backend-file mappings
> (MAP_SHARED). GUP rejects long-term writable pins on dirty tracked file
> mappings since:
> 
> commit 8ac268436e6d ("mm/gup: disallow FOLL_LONGTERM GUP-nonfast writing to file-backed mappings")
> commit a6e79df92e4a ("mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings").
> 
> Region registration only requires long-term pin to prevent page migration and
> does not write through this GUP pin.
> 
> Drop FOLL_WRITE and pin guest memory only with FOLL_LONGTERM.

Worth mentioning here something like

"In the past, FOLL_WRITE was required to trigger CoW unsharing, making sure that
we don't end up replacing the page in the page tables during a later write fault
after already having pinned a (shared) page in MAP_PRIVATE mappings.
FOLL_LONGTERM does that nowadays (see gup_must_unshare()) even without FOLL_WRITE.

Given that SEV only pins RAM for XYZ and doesn't actually write to the pinned
pages, we can just drop the FOLL_WRITE"

Fill out XYZ :)

In general, LGTM

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David

Re: [PATCH v2] KVM: SEV: drop FOLL_WRITE for encrypted region registration

[Gupta, Pankaj]

>> Commit 7e066cb9b71a ("KVM: SEV: Use long-term pin when registering
>> encrypted memory regions") added FOLL_LONGTERM to
>> sev_mem_enc_register_region() so anonymous guest RAM is migrated out of
>> MIGRATE_CMA/ZONE_MOVABLE before a long term pin. It also kept
>> FOLL_WRITE on the pin.
>>
>> Combining FOLL_WRITE with FOLL_LONGTERM breaks registration of file-backed
>> guest memory, such as virtio-pmem host memory-backend-file mappings
>> (MAP_SHARED). GUP rejects long-term writable pins on dirty tracked file
>> mappings since:
>>
>> commit 8ac268436e6d ("mm/gup: disallow FOLL_LONGTERM GUP-nonfast writing to file-backed mappings")
>> commit a6e79df92e4a ("mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings").
>>
>> Region registration only requires long-term pin to prevent page migration and
>> does not write through this GUP pin.
>>
>> Drop FOLL_WRITE and pin guest memory only with FOLL_LONGTERM.
> Worth mentioning here something like
>
> "In the past, FOLL_WRITE was required to trigger CoW unsharing, making sure that
> we don't end up replacing the page in the page tables during a later write fault
> after already having pinned a (shared) page in MAP_PRIVATE mappings.
> FOLL_LONGTERM does that nowadays (see gup_must_unshare()) even without FOLL_WRITE.
>
> Given that SEV only pins RAM for XYZ and doesn't actually write to the pinned
> pages, we can just drop the FOLL_WRITE"
>
> Fill out XYZ :)

Sure :)

I will update the commit message in v3.

>
> In general, LGTM
>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>

thanks

Pankaj