linux-kernel - Re: [PATCH v2 3/3] kvm/x86: Allow to respond to generic signals during slow page faults

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Date:   Thu, 11 Aug 2022 16:58:07 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        David Hildenbrand <david@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Linux MM Mailing List <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH v2 3/3] kvm/x86: Allow to respond to generic signals
 during slow page faults

On Thu, Aug 11, 2022 at 08:12:38PM +0000, Sean Christopherson wrote:
> On Wed, Jul 20, 2022, Peter Xu wrote:
> > All the facilities should be ready for this, what we need to do is to add a
> > new "interruptible" flag showing that we're willing to be interrupted by
> > common signals during the __gfn_to_pfn_memslot() request, and wire it up
> > with a FOLL_INTERRUPTIBLE flag that we've just introduced.
> > 
> > Note that only x86 slow page fault routine will set this to true.  The new
> > flag is by default false in non-x86 arch or on other gup paths even for
> > x86.  It can actually be used elsewhere too but not yet covered.
> > 
> > When we see the PFN fetching was interrupted, do early exit to userspace
> > with an KVM_EXIT_INTR exit reason.
> > 
> > Signed-off-by: Peter Xu <peterx@...hat.com>
> > ---
> >  arch/arm64/kvm/mmu.c                   |  2 +-
> >  arch/powerpc/kvm/book3s_64_mmu_hv.c    |  2 +-
> >  arch/powerpc/kvm/book3s_64_mmu_radix.c |  2 +-
> >  arch/x86/kvm/mmu/mmu.c                 | 16 ++++++++++++--
> >  include/linux/kvm_host.h               |  4 ++--
> >  virt/kvm/kvm_main.c                    | 30 ++++++++++++++++----------
> >  virt/kvm/kvm_mm.h                      |  4 ++--
> >  virt/kvm/pfncache.c                    |  2 +-
> >  8 files changed, 41 insertions(+), 21 deletions(-)
> 
> I don't usually like adding code without a user, but in this case I think I'd
> prefer to add the @interruptible param and then activate x86's kvm_faultin_pfn()
> in a separate patch.  It's rather difficult to tease out the functional x86
> change, and that would also allow other architectures to use the interruptible
> support without needing to depend on the functional x86 change.
> 
> And maybe squash the addition of @interruptible with the previous patch?  I.e.
> add all of the infrastructure for KVM_PFN_ERR_SIGPENDING in patch 2, then use it
> in x86 in patch 3.

Sounds good.

> 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 17252f39bd7c..aeafe0e9cfbf 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -3012,6 +3012,13 @@ static int kvm_handle_bad_page(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn)
> >  static int handle_abnormal_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
> >  			       unsigned int access)
> >  {
> > +	/* NOTE: not all error pfn is fatal; handle sigpending pfn first */
> > +	if (unlikely(is_sigpending_pfn(fault->pfn))) {
> 
> Move this into kvm_handle_bad_page(), then there's no need for a comment to call
> out that this needs to come before the is_error_pfn() check.  This _is_ a "bad"
> PFN, it just so happens that userspace might be able to resolve the "bad" PFN.

It's a pity it needs to be in "bad pfn" category since that's the only
thing we can easily use, but true it is now.

> 
> > +		vcpu->run->exit_reason = KVM_EXIT_INTR;
> > +		++vcpu->stat.signal_exits;
> > +		return -EINTR;
> 
> For better or worse, kvm_handle_signal_exit() exists and can be used here.  I
> don't love that KVM details bleed into xfer_to_guest_mode_work(), but that's a
> future problem.
> 
> I do think that the "return -EINTR" should be moved into kvm_handle_signal_exit(),
> partly for code reuse and partly because returning -EINTR is very much KVM ABI.
> Oof, but there are a _lot_ of paths that can use kvm_handle_signal_exit(), and
> some of them don't select KVM_XFER_TO_GUEST_WORK, i.e. kvm_handle_signal_exit()
> should be defined unconditionally.  I'll work on a series to handle that separately,
> no reason to take a dependency on that cleanup.
> 
> So for now,
> 
> static int kvm_handle_bad_page(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn)
> {
> 	if (pfn == KVM_PFN_ERR_SIGPENDING) {
> 		kvm_handle_signal_exit(vcpu);
> 		return -EINTR;
> 	}
> 
> 	...
> }

Sounds good too here.  Also all points taken in the wording of patch 2.

Will respin shortly, thanks Sean.

-- 
Peter Xu