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[PATCH v4 1/2] KVM: guest_memfd: add generic population via write

Kalyazin, Nikita posted 2 patches 1 month ago
There is a newer version of this series
[PATCH v4 1/2] KVM: guest_memfd: add generic population via write
Posted by Kalyazin, Nikita 1 month ago 
write syscall populates guest_memfd with user-supplied data in a generic
way, ie no vendor-specific preparation is performed.  This is supposed
to be used in non-CoCo setups where guest memory is not
hardware-encrypted.

The following behaviour is implemented:
 - only page-aligned count and offset are allowed
 - if the memory is already allocated, the call will successfully
   populate it
 - if the memory is not allocated, the call will both allocate and
   populate
 - if the memory is already populated, the call will not repopulate it

Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
---
 virt/kvm/guest_memfd.c | 64 +++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 63 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 08a6bc7d25b6..1f6f85edace0 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -379,7 +379,9 @@ static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
 }
 
 static struct file_operations kvm_gmem_fops = {
-	.mmap		= kvm_gmem_mmap,
+	.mmap           = kvm_gmem_mmap,
+	.llseek         = default_llseek,
+	.write_iter     = generic_perform_write,
 	.open		= generic_file_open,
 	.release	= kvm_gmem_release,
 	.fallocate	= kvm_gmem_fallocate,
@@ -390,6 +392,63 @@ void kvm_gmem_init(struct module *module)
 	kvm_gmem_fops.owner = module;
 }
 
+static int kvm_kmem_gmem_write_begin(const struct kiocb *kiocb,
+				     struct address_space *mapping,
+				     loff_t pos, unsigned len,
+				     struct folio **foliop,
+				     void **fsdata)
+{
+	struct file *file = kiocb->ki_filp;
+	pgoff_t index = pos >> PAGE_SHIFT;
+	struct folio *folio;
+
+	if (!PAGE_ALIGNED(pos) || len != PAGE_SIZE)
+		return -EINVAL;
+
+	if (pos + len > i_size_read(file_inode(file)))
+		return -EINVAL;
+
+	folio = kvm_gmem_get_folio(file_inode(file), index);
+	if (IS_ERR(folio))
+		return -EFAULT;
+
+	if (WARN_ON_ONCE(folio_test_large(folio))) {
+		folio_unlock(folio);
+		folio_put(folio);
+		return -EFAULT;
+	}
+
+	if (folio_test_uptodate(folio)) {
+		folio_unlock(folio);
+		folio_put(folio);
+		return -ENOSPC;
+	}
+
+	*foliop = folio;
+	return 0;
+}
+
+static int kvm_kmem_gmem_write_end(const struct kiocb *kiocb,
+				   struct address_space *mapping,
+				   loff_t pos, unsigned len, unsigned copied,
+				   struct folio *folio, void *fsdata)
+{
+	int ret;
+
+	if (copied == len) {
+		kvm_gmem_mark_prepared(folio);
+		ret = copied;
+	} else {
+		filemap_remove_folio(folio);
+		ret = 0;
+	}
+
+	folio_unlock(folio);
+	folio_put(folio);
+
+	return ret;
+}
+
 static int kvm_gmem_migrate_folio(struct address_space *mapping,
 				  struct folio *dst, struct folio *src,
 				  enum migrate_mode mode)
@@ -442,6 +501,8 @@ static void kvm_gmem_free_folio(struct folio *folio)
 
 static const struct address_space_operations kvm_gmem_aops = {
 	.dirty_folio = noop_dirty_folio,
+	.write_begin = kvm_kmem_gmem_write_begin,
+	.write_end = kvm_kmem_gmem_write_end,
 	.migrate_folio	= kvm_gmem_migrate_folio,
 	.error_remove_folio = kvm_gmem_error_folio,
 #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_INVALIDATE
@@ -489,6 +550,7 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
 	}
 
 	file->f_flags |= O_LARGEFILE;
+	file->f_mode |= FMODE_LSEEK | FMODE_PWRITE;
 
 	inode = file->f_inode;
 	WARN_ON(file->f_mapping != inode->i_mapping);
-- 
2.50.1
Re: [PATCH v4 1/2] KVM: guest_memfd: add generic population via write
Posted by kernel test robot 1 month ago 
Hi Nikita,

kernel test robot noticed the following build errors:

[auto build test ERROR on a6ad54137af92535cfe32e19e5f3bc1bb7dbd383]

url:    https://github.com/intel-lab-lkp/linux/commits/Kalyazin-Nikita/KVM-guest_memfd-add-generic-population-via-write/20250828-233437
base:   a6ad54137af92535cfe32e19e5f3bc1bb7dbd383
patch link:    https://lore.kernel.org/r/20250828153049.3922-2-kalyazin%40amazon.com
patch subject: [PATCH v4 1/2] KVM: guest_memfd: add generic population via write
config: x86_64-randconfig-001-20250830 (https://download.01.org/0day-ci/archive/20250831/202508310252.E5uFh1hx-lkp@intel.com/config)
compiler: clang version 20.1.8 (https://github.com/llvm/llvm-project 87f0227cb60147a26a1eeb4fb06e3b505e9c7261)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250831/202508310252.E5uFh1hx-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202508310252.E5uFh1hx-lkp@intel.com/

All errors (new ones prefixed by >>, old ones prefixed by <<):

>> ERROR: modpost: "filemap_remove_folio" [arch/x86/kvm/kvm.ko] undefined!

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki
Re: [PATCH v4 1/2] KVM: guest_memfd: add generic population via write
Posted by David Hildenbrand 1 month ago 
On 30.08.25 20:56, kernel test robot wrote:
> Hi Nikita,
> 
> kernel test robot noticed the following build errors:
> 
> [auto build test ERROR on a6ad54137af92535cfe32e19e5f3bc1bb7dbd383]
> 
> url:    https://github.com/intel-lab-lkp/linux/commits/Kalyazin-Nikita/KVM-guest_memfd-add-generic-population-via-write/20250828-233437
> base:   a6ad54137af92535cfe32e19e5f3bc1bb7dbd383
> patch link:    https://lore.kernel.org/r/20250828153049.3922-2-kalyazin%40amazon.com
> patch subject: [PATCH v4 1/2] KVM: guest_memfd: add generic population via write
> config: x86_64-randconfig-001-20250830 (https://download.01.org/0day-ci/archive/20250831/202508310252.E5uFh1hx-lkp@intel.com/config)
> compiler: clang version 20.1.8 (https://github.com/llvm/llvm-project 87f0227cb60147a26a1eeb4fb06e3b505e9c7261)
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250831/202508310252.E5uFh1hx-lkp@intel.com/reproduce)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202508310252.E5uFh1hx-lkp@intel.com/
> 
> All errors (new ones prefixed by >>, old ones prefixed by <<):
> 
>>> ERROR: modpost: "filemap_remove_folio" [arch/x86/kvm/kvm.ko] undefined!

Right, that one is not exported.

-- 
Cheers

David / dhildenb
Re: [PATCH v4 1/2] KVM: guest_memfd: add generic population via write
Posted by Nikita Kalyazin 1 month ago 

On 01/09/2025 11:09, David Hildenbrand wrote:

> On 30.08.25 20:56, kernel test robot wrote:
>> Hi Nikita,
>>
>> kernel test robot noticed the following build errors:
>>
>> [auto build test ERROR on a6ad54137af92535cfe32e19e5f3bc1bb7dbd383]
>>
>> url:    https://github.com/intel-lab-lkp/linux/commits/Kalyazin- 
>> Nikita/KVM-guest_memfd-add-generic-population-via-write/20250828-233437
>> base:   a6ad54137af92535cfe32e19e5f3bc1bb7dbd383
>> patch link:    https://lore.kernel.org/r/20250828153049.3922-2- 
>> kalyazin%40amazon.com
>> patch subject: [PATCH v4 1/2] KVM: guest_memfd: add generic population 
>> via write
>> config: x86_64-randconfig-001-20250830 (https://download.01.org/0day- 
>> ci/archive/20250831/202508310252.E5uFh1hx-lkp@intel.com/config)
>> compiler: clang version 20.1.8 (https://github.com/llvm/llvm-project 
>> 87f0227cb60147a26a1eeb4fb06e3b505e9c7261)
>> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/ 
>> archive/20250831/202508310252.E5uFh1hx-lkp@intel.com/reproduce)
>>
>> If you fix the issue in a separate patch/commit (i.e. not just a new 
>> version of
>> the same patch/commit), kindly add following tags
>> | Reported-by: kernel test robot <lkp@intel.com>
>> | Closes: https://lore.kernel.org/oe-kbuild-all/202508310252.E5uFh1hx- 
>> lkp@intel.com/
>>
>> All errors (new ones prefixed by >>, old ones prefixed by <<):
>>
>>>> ERROR: modpost: "filemap_remove_folio" [arch/x86/kvm/kvm.ko] undefined!
> 
> Right, that one is not exported.

Indeed.  Having a look at other .write_end implementations, it looks 
like the convention is zeroing the uncopied part instead of trying to 
remove the folio from the cache.  Will do it in the next version.

> 
> -- 
> Cheers
> 
> David / dhildenb
>
Re: [PATCH v4 1/2] KVM: guest_memfd: add generic population via write
Posted by David Hildenbrand 1 month ago 
On 28.08.25 17:31, Kalyazin, Nikita wrote:
> write syscall populates guest_memfd with user-supplied data in a generic
> way, ie no vendor-specific preparation is performed.  This is supposed
> to be used in non-CoCo setups where guest memory is not
> hardware-encrypted.
> 
> The following behaviour is implemented:
>   - only page-aligned count and offset are allowed
>   - if the memory is already allocated, the call will successfully
>     populate it
>   - if the memory is not allocated, the call will both allocate and
>     populate
>   - if the memory is already populated, the call will not repopulate it
> 
> Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
> ---

Just nothing that checkpatch complains about

a) Usage of "unsigned" instead of "unsigned int"

b) The From doesn't completely match the SOB: "Kalyazin, Nikita" vs 
"Nikita Kalyazin"

-- 
Cheers

David / dhildenb
Re: [PATCH v4 1/2] KVM: guest_memfd: add generic population via write
Posted by Nikita Kalyazin 1 month ago 

On 28/08/2025 21:01, David Hildenbrand wrote:
> On 28.08.25 17:31, Kalyazin, Nikita wrote:
>> write syscall populates guest_memfd with user-supplied data in a generic
>> way, ie no vendor-specific preparation is performed.  This is supposed
>> to be used in non-CoCo setups where guest memory is not
>> hardware-encrypted.
>>
>> The following behaviour is implemented:
>>   - only page-aligned count and offset are allowed
>>   - if the memory is already allocated, the call will successfully
>>     populate it
>>   - if the memory is not allocated, the call will both allocate and
>>     populate
>>   - if the memory is already populated, the call will not repopulate it
>>
>> Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
>> ---
> 
> Just nothing that checkpatch complains about
> 
> a) Usage of "unsigned" instead of "unsigned int"

Hi David,


I copied the prototypes straight from the fs.h...  In any case, will fix 
in the next version.

> 
> b) The From doesn't completely match the SOB: "Kalyazin, Nikita" vs
> "Nikita Kalyazin"

It's about .com vs .co.uk, I think.  Will have to use "From:" apparently.


Thanks,
Nikita

> 
> -- 
> Cheers
> 
> David / dhildenb
>
Re: [PATCH v4 1/2] KVM: guest_memfd: add generic population via write
Posted by David Hildenbrand 1 month ago 
On 01.09.25 16:29, Nikita Kalyazin wrote:
> 
> 
> On 28/08/2025 21:01, David Hildenbrand wrote:
>> On 28.08.25 17:31, Kalyazin, Nikita wrote:
>>> write syscall populates guest_memfd with user-supplied data in a generic
>>> way, ie no vendor-specific preparation is performed.  This is supposed
>>> to be used in non-CoCo setups where guest memory is not
>>> hardware-encrypted.
>>>
>>> The following behaviour is implemented:
>>>    - only page-aligned count and offset are allowed
>>>    - if the memory is already allocated, the call will successfully
>>>      populate it
>>>    - if the memory is not allocated, the call will both allocate and
>>>      populate
>>>    - if the memory is already populated, the call will not repopulate it
>>>
>>> Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
>>> ---
>>
>> Just nothing that checkpatch complains about
>>
>> a) Usage of "unsigned" instead of "unsigned int"
> 
> Hi David,
> 
> 
> I copied the prototypes straight from the fs.h...  In any case, will fix
> in the next version.

Yes, realized that after I sent it. :)

> 
>>
>> b) The From doesn't completely match the SOB: "Kalyazin, Nikita" vs
>> "Nikita Kalyazin"
> 
> It's about .com vs .co.uk, I think.  Will have to use "From:" apparently.

Yes, discussed that with Patrick on the other thread: sending from .com 
is apparently fine.

-- 
Cheers

David / dhildenb

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