linux-kernel - Re: [PATCH 09/11] KVM: guest_memfd: Add interface for populating gmem pages with user data

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Date: Thu, 25 Apr 2024 08:01:25 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Isaku Yamahata <isaku.yamahata@...el.com>
Cc: Sean Christopherson <seanjc@...gle.com>, linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	michael.roth@....com, isaku.yamahata@...ux.intel.com
Subject: Re: [PATCH 09/11] KVM: guest_memfd: Add interface for populating gmem
 pages with user data

On Thu, Apr 25, 2024 at 3:12 AM Isaku Yamahata <isaku.yamahata@...el.com> wrote:
> > >   get_user_pages_fast(source addr)
> > >   read_lock(mmu_lock)
> > >   kvm_tdp_mmu_get_walk_private_pfn(vcpu, gpa, &pfn);
> > >   if the page table doesn't map gpa, error.
> > >   TDH.MEM.PAGE.ADD()
> > >   TDH.MR.EXTEND()
> > >   read_unlock(mmu_lock)
> > >   put_page()
> >
> > Hmm, KVM doesn't _need_ to use invalidate_lock to protect against guest_memfd
> > invalidation, but I also don't see why it would cause problems.

The invalidate_lock is only needed to operate on the guest_memfd, but
it's a rwsem so there are no risks of lock inversion.

> > I.e. why not
> > take mmu_lock() in TDX's post_populate() implementation?
>
> We can take the lock.  Because we have already populated the GFN of guest_memfd,
> we need to make kvm_gmem_populate() not pass FGP_CREAT_ONLY.  Otherwise we'll
> get -EEXIST.

I don't understand why TDH.MEM.PAGE.ADD() cannot be called from the
post-populate hook. Can the code for TDH.MEM.PAGE.ADD be shared
between the memory initialization ioctl and the page fault hook in
kvm_x86_ops?

Paolo

>
> > That would allow having
> > a sanity check that the PFN that guest_memfd() has is indeed the PFN that KVM's
> > S-EPT mirror has, i.e. the PFN that KVM is going to PAGE.ADD.
>
> Because we have PFN from the mirrored EPT, I thought it's duplicate to get PFN
> again via guest memfd.  We can check if two PFN matches.
> --
> Isaku Yamahata <isaku.yamahata@...el.com>
>