Message-ID: <CADrL8HUObfEd80sr783dB3dPWGSX7H5=0HCp9OjiL6D_Sp+2Ww@mail.gmail.com>
Date: Fri, 12 Sep 2025 15:34:06 -0700
From: James Houghton <jthoughton@...gle.com>
To: kalyazin@...zon.com
Cc: "Kalyazin, Nikita" <kalyazin@...zon.co.uk>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"shuah@...nel.org" <shuah@...nel.org>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "michael.day@....com" <michael.day@....com>, 
	"david@...hat.com" <david@...hat.com>, "Roy, Patrick" <roypat@...zon.co.uk>, 
	"Thomson, Jack" <jackabt@...zon.co.uk>, "Manwaring, Derek" <derekmn@...zon.com>, 
	"Cali, Marco" <xmarcalx@...zon.co.uk>
Subject: Re: [PATCH v5 1/2] KVM: guest_memfd: add generic population via write

On Thu, Sep 11, 2025 at 3:15 AM Nikita Kalyazin <kalyazin@...zon.com> wrote:
>
>
>
> On 10/09/2025 22:23, James Houghton wrote:
> > On Tue, Sep 2, 2025 at 4:20 AM Kalyazin, Nikita <kalyazin@...zon.co.uk> wrote:
> >>
> >> From: Nikita Kalyazin <kalyazin@...zon.com>
> >
> > Hi Nikita,
>
> Hi James,
>
> Thanks for the review!

:) I hope it's actually helpful.

>
>
> >>
> >> 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.
> >
> > What's meant to happen if we do use this for CoCo VMs? I would expect
> > write() to fail, but I don't see why it would (seems like we need/want
> > a check that we aren't write()ing to private memory).
>
> I am not so sure that write() should fail even in CoCo VMs if we access
> not-yet-prepared pages.  My understanding was that the CoCoisation of
> the memory occurs during "preparation".  But I may be wrong here.

This sounds fine to me, but could you update the changelog with what
the behavior is for CoCo VMs and why we don't allow writing to the
same pages/regions twice? Something like:

"Although write() is only meant to be used for non-CoCo VMs, it is
valid for CoCo VMs as well: the written contents will be encrypted
(when the page is prepared). Because the contents may be encrypted, it
is nonsensical to write() again, so write()ing to prepared pages is
disallowed (even if the no memory encryption occurs). Furthermore, in
the near future, page preparation will also result in pages being
removed from the direct map, so there will be no direct map through
which to perform the second write()."

(This is all provided that it's actually okay to write() content that
will be encrypted... I don't know why that would be improper, but I'm
not exactly an expert here.)

> >> @@ -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 int 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;
> >
> > Requiring pos to be page-aligned seems like a strange restriction, and
> > requiring len to be exactly PAGE_SIZE just seems wrong. I don't see
> > any reason why the below logic can't be made to work with an
> > unrestricted pos and len (in other words, I don't see how guest_memfd
> > is special vs other filesystems in this regard).
>
> I don't have a real reason to apply those restrictions.  Happy to remove
> them, thanks.

Thanks! Presumably you'll make it so that any unaligned segments will
be left as zeroes; please describe this in the changelog as well. :)

> >> +
> >> +       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;
> >
> > Does it actually matter for the folio not to be uptodate? It seems
> > unnecessarily restrictive not to be able to overwrite data if we're
> > saying that this is only usable for unencrypted memory anyway.
>
> In the context of direct map removal [1] it does actually because when
> we mark a folio as prepared, we remove it from the direct map making it
> inaccessible to the way write() performs the copy.  It does not matter
> if direct map removal isn't enabled though.  Do you think it should be
> conditional?

Oh, good point. It's simpler (both to implement and to describe) to
disallow a second write() call in all cases (no matter if the direct
map for the page has been removed or if the contents have been
encrypted), so I'm all for leaving it unconditional like you have now.
Thanks!

>
> [1]: https://lore.kernel.org/kvm/20250828093902.2719-1-roypat@amazon.co.uk
>
> >
> > Is ENOSPC really the right errno for this? (Maybe -EFAULT?)
>
> I don't have a strong opinion here.  My reasoning was if the folio is
> already "sealed" by the direct map removal, then it is no longer a part
> of the "writable space", so -ENOSPC makes sense.  Maybe this intuition
> only works for me so I'm happy to change to -EFAULT if it looks less
> confusing.

Oh actually.... how about EEXIST? That feels like the most natural.
But also no strong opinion here.

Thanks for all the clarification, Nikita. :)

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