Re: [PATCH 4/4] kvm/x86: Allow to respond to generic signals during slow page faults

[Sean Christopherson <seanjc@google.com>]

On Thu, Jun 23, 2022, Peter Xu wrote:
> Hi, Sean,
> 
> On Thu, Jun 23, 2022 at 02:46:08PM +0000, Sean Christopherson wrote:
> > On Wed, Jun 22, 2022, Peter Xu wrote:
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index e92f1ab63d6a..b39acb7cb16d 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 intr before the other ones */
> > > +	if (unlikely(is_intr_pfn(fault->pfn))) {
> > > +		vcpu->run->exit_reason = KVM_EXIT_INTR;
> > > +		++vcpu->stat.signal_exits;
> > > +		return -EINTR;
> > > +	}
> > > +
> > >  	/* The pfn is invalid, report the error! */
> > >  	if (unlikely(is_error_pfn(fault->pfn)))
> > >  		return kvm_handle_bad_page(vcpu, fault->gfn, fault->pfn);
> > > @@ -4017,6 +4024,8 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> > >  		}
> > >  	}
> > >  
> > > +	/* Allow to respond to generic signals in slow page faults */
> > 
> > "slow" is being overloaded here.  The previous call __gfn_to_pfn_memslot() will
> > end up in hva_to_pfn_slow(), but because of passing a non-null async it won't wait.
> > This code really should have a more extensive comment irrespective of the interruptible
> > stuff, now would be a good time to add that.
> 
> Yes I agree, especially the "async" parameter along with "atomic" makes it
> even more confusing as you said.  But isn't that also means the "slow" here
> is spot-on?  I mean imho it's the "elsewhere" needs cleanup not this
> comment itself since it's really stating the fact that this is the real
> slow path?

No, because atomic=false here, i.e. KVM will try hva_to_pfn_slow() if hva_to_pfn_fast()
fails.  So even if we agree that the "wait for IO" path is the true slow path,
when reading KVM code the vast, vast majority of developers will associate "slow"
with hva_to_pfn_slow().

> Or any other suggestions greatly welcomed on how I should improve this
> comment.

Something along these lines?

	/*
	 * Allow gup to bail on pending non-fatal signals when it's also allowed
	 * to wait for IO.  Note, gup always bails if it is unable to quickly
	 * get a page and a fatal signal, i.e. SIGKILL, is pending.
	 */
> 
> > 
> > Comments aside, isn't this series incomplete from the perspective that there are
> > still many flows where KVM will hang if gfn_to_pfn() gets stuck in gup?  E.g. if
> > KVM is retrieving a page pointed at by vmcs12.
> 
> Right.  The thing is I'm not confident I can make it complete easily in one
> shot..
> 
> I mentioned some of that in cover letter or commit message of patch 1, in
> that I don't think all the gup call sites are okay with being interrupted
> by a non-fatal signal.
> 
> So what I want to do is doing it step by step, at least by introducing
> FOLL_INTERRUPTIBLE and having one valid user of it that covers a very valid
> use case.  I'm also pretty sure the page fault path is really the most
> cases that will happen with GUP, so it already helps in many ways for me
> when running with a patched kernel.
> 
> So when the complete picture is non-trivial to achieve in one shot, I think
> this could be one option we go for.  With the facility (and example code on
> x86 slow page fault) ready, hopefully we could start to convert many other
> call sites to be signal-aware, outside page faults, or even outside x86,
> because it's really a more generic problem, which I fully agree.
> 
> Does it sound reasonable to you?

Yep.  In fact, I'd be totally ok keeping this to just the page fault path.  I
missed one cruicial detail on my first read through: gup already bails on SIGKILL,
it's only these technically-not-fatal signals that gup ignores by default.  In
other words, using FOLL_INTERRUPTIBLE is purely opportunsitically as userspace
can always resort to SIGKILL if the VM really needs to die.

It would be very helpful to explicit call that out in the changelog, that way
it's (hopefully) clear that KVM uses FOLL_INTERRUPTIBLE to be user friendly when
it's easy to do so, and that it's not required for correctness/robustness.