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

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Message-ID: <CABgObfaxAd_J5ufr+rOcND=-NWrOzVsvavoaXuFw_cwDd+e9aA@mail.gmail.com>
Date: Fri, 26 Apr 2024 07:41:42 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Isaku Yamahata <isaku.yamahata@...el.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 6:00 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Thu, Apr 25, 2024, Paolo Bonzini wrote:
> > 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?
>
> Ah, because TDX is required to pre-fault the memory to establish the S-EPT walk,
> and pre-faulting means guest_memfd()
>
> Requiring that guest_memfd not have a page when initializing the guest image
> seems wrong, i.e. I don't think we want FGP_CREAT_ONLY.  And not just because I
> am a fan of pre-faulting, I think the semantics are bad.

Ok, fair enough. I wanted to do the once-only test in common code but
since SEV code checks for the RMP I can remove that. One less
headache.

Paolo