Date: Fri, 10 May 2024 11:37:19 -0500
From: Michael Roth <michael.roth@....com>
To: Paolo Bonzini <pbonzini@...hat.com>
CC: Sean Christopherson <seanjc@...gle.com>, <kvm@...r.kernel.org>,
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Subject: Re: [PATCH v15 22/23] KVM: SEV: Fix return code interpretation for
 RMP nested page faults

On Fri, May 10, 2024 at 06:01:52PM +0200, Paolo Bonzini wrote:
> On 5/10/24 15:58, Sean Christopherson wrote:
> > On Thu, May 09, 2024, Michael Roth wrote:
> > > The intended logic when handling #NPFs with the RMP bit set (31) is to
> > > first check to see if the #NPF requires a shared<->private transition
> > > and, if so, to go ahead and let the corresponding KVM_EXIT_MEMORY_FAULT
> > > get forwarded on to userspace before proceeding with any handling of
> > > other potential RMP fault conditions like needing to PSMASH the RMP
> > > entry/etc (which will be done later if the guest still re-faults after
> > > the KVM_EXIT_MEMORY_FAULT is processed by userspace).
> > > 
> > > The determination of whether any userspace handling of
> > > KVM_EXIT_MEMORY_FAULT is needed is done by interpreting the return code
> > > of kvm_mmu_page_fault(). However, the current code misinterprets the
> > > return code, expecting 0 to indicate a userspace exit rather than less
> > > than 0 (-EFAULT). This leads to the following unexpected behavior:
> > > 
> > >    - for KVM_EXIT_MEMORY_FAULTs resulting for implicit shared->private
> > >      conversions, warnings get printed from sev_handle_rmp_fault()
> > >      because it does not expect to be called for GPAs where
> > >      KVM_MEMORY_ATTRIBUTE_PRIVATE is not set. Standard linux guests don't
> > >      generally do this, but it is allowed and should be handled
> > >      similarly to private->shared conversions rather than triggering any
> > >      sort of warnings
> > > 
> > >    - if gmem support for 2MB folios is enabled (via currently out-of-tree
> > >      code), implicit shared<->private conversions will always result in
> > >      a PSMASH being attempted, even if it's not actually needed to
> > >      resolve the RMP fault. This doesn't cause any harm, but results in a
> > >      needless PSMASH and zapping of the sPTE
> > > 
> > > Resolve these issues by calling sev_handle_rmp_fault() only when
> > > kvm_mmu_page_fault()'s return code is greater than or equal to 0,
> > > indicating a KVM_MEMORY_EXIT_FAULT/-EFAULT isn't needed. While here,
> > > simplify the code slightly and fix up the associated comments for better
> > > clarity.
> > > 
> > > Fixes: ccc9d836c5c3 ("KVM: SEV: Add support to handle RMP nested page faults")
> > > 
> > > Signed-off-by: Michael Roth <michael.roth@....com>
> > > ---
> > >   arch/x86/kvm/svm/svm.c | 10 ++++------
> > >   1 file changed, 4 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > > index 426ad49325d7..9431ce74c7d4 100644
> > > --- a/arch/x86/kvm/svm/svm.c
> > > +++ b/arch/x86/kvm/svm/svm.c
> > > @@ -2070,14 +2070,12 @@ static int npf_interception(struct kvm_vcpu *vcpu)
> > >   				svm->vmcb->control.insn_len);
> > >   	/*
> > > -	 * rc == 0 indicates a userspace exit is needed to handle page
> > > -	 * transitions, so do that first before updating the RMP table.
> > > +	 * rc < 0 indicates a userspace exit may be needed to handle page
> > > +	 * attribute updates, so deal with that first before handling other
> > > +	 * potential RMP fault conditions.
> > >   	 */
> > > -	if (error_code & PFERR_GUEST_RMP_MASK) {
> > > -		if (rc == 0)
> > > -			return rc;
> > > +	if (rc >= 0 && error_code & PFERR_GUEST_RMP_MASK)
> > 
> > This isn't correct either.  A return of '0' also indiciates "exit to userspace",
> > it just doesn't happen with SNP because '0' is returned only when KVM attempts
> > emulation, and that too gets short-circuited by svm_check_emulate_instruction().
> > 
> > And I would honestly drop the comment, KVM's less-than-pleasant 1/0/-errno return
> > values overload is ubiquitous enough that it should be relatively self-explanatory.
> > 
> > Or if you prefer to keep a comment, drop the part that specifically calls out
> > attributes updates, because that incorrectly implies that's the _only_ reason
> > why KVM checks the return.  But my vote is to drop the comment, because it
> > essentially becomes "don't proceed to step 2 if step 1 failed", which kind of
> > makes the reader go "well, yeah".
> 
> So IIUC you're suggesting
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 426ad49325d7..c39eaeb21981 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -2068,16 +2068,11 @@ static int npf_interception(struct kvm_vcpu *vcpu)
>  				static_cpu_has(X86_FEATURE_DECODEASSISTS) ?
>  				svm->vmcb->control.insn_bytes : NULL,
>  				svm->vmcb->control.insn_len);
> +	if (rc <= 0)
> +		return rc;
> -	/*
> -	 * rc == 0 indicates a userspace exit is needed to handle page
> -	 * transitions, so do that first before updating the RMP table.
> -	 */
> -	if (error_code & PFERR_GUEST_RMP_MASK) {
> -		if (rc == 0)
> -			return rc;
> +	if (error_code & PFERR_GUEST_RMP_MASK)
>  		sev_handle_rmp_fault(vcpu, fault_address, error_code);
> -	}
>  	return rc;
>  }
> 
> ?
> 
> So, we're... a bit tight for 6.10 to include SNP and that is an
> understatement.  My plan is to merge it for 6.11, but do so
> immediately after the merge window ends.  In other words, it
> is a delay in terms of release but not in terms of time.  I
> don't want QEMU and kvm-unit-tests work to be delayed any
> further, in particular.

That's unfortunate, I'd thought from the PUCK call that we still had
some time to stabilize things before merge window. But whatever you
think is best.

> 
> Once we sort out the loose ends of patches 21-23, you could send
> it as a pull request.

Ok, as a pull request against kvm/next, or kvm/queue?

Thanks,

Mike

> 
> Paolo
> 
> 

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