linux-kernel - Re: [PATCH 3/4] kvm: Add new pfn error KVM_PFN_ERR

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Message-ID: <YrS/13dBmSIpvd3C@xz-m1.local>
Date:   Thu, 23 Jun 2022 15:32:39 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linux MM Mailing List <linux-mm@...ck.org>
Subject: Re: [PATCH 3/4] kvm: Add new pfn error KVM_PFN_ERR_INTR

On Thu, Jun 23, 2022 at 02:31:47PM +0000, Sean Christopherson wrote:
> On Wed, Jun 22, 2022, Peter Xu wrote:
> > Add one new PFN error type to show when we cannot finish fetching the PFN
> > due to interruptions.  For example, by receiving a generic signal.
> > 
> > This prepares KVM to be able to respond to SIGUSR1 (for QEMU that's the
> > SIGIPI) even during e.g. handling an userfaultfd page fault.
> > 
> > Signed-off-by: Peter Xu <peterx@...hat.com>
> > ---
> >  include/linux/kvm_host.h | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index b646b6fcaec6..4f84a442f67f 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -96,6 +96,7 @@
> >  #define KVM_PFN_ERR_FAULT	(KVM_PFN_ERR_MASK)
> >  #define KVM_PFN_ERR_HWPOISON	(KVM_PFN_ERR_MASK + 1)
> >  #define KVM_PFN_ERR_RO_FAULT	(KVM_PFN_ERR_MASK + 2)
> > +#define KVM_PFN_ERR_INTR	(KVM_PFN_ERR_MASK + 3)
> >  
> >  /*
> >   * error pfns indicate that the gfn is in slot but faild to
> > @@ -106,6 +107,16 @@ static inline bool is_error_pfn(kvm_pfn_t pfn)
> >  	return !!(pfn & KVM_PFN_ERR_MASK);
> >  }
> >  
> > +/*
> > + * When KVM_PFN_ERR_INTR is returned, it means we're interrupted during
> > + * fetching the PFN (e.g. a signal might have arrived), so we may want to
> > + * retry at some later point and kick the userspace to handle the signal.
> > + */
> > +static inline bool is_intr_pfn(kvm_pfn_t pfn)
> > +{
> > +	return pfn == KVM_PFN_ERR_INTR;
> 
> What about is_sigpending_pfn() and KVM_PFN_ERR_SIGPENDING?  "intr" is too close to
> a real thing KVM will encounter, and I think knowing that KVM is effectively
> responding to a pending signal is the most important detail for KVM developers
> encountering this code for this first time.  E.g. from KVM_PFN_ERR_INTR alone, one
> might think that any interrupt during GUP will trigger this.

Sounds good; INTR could be too general for KVM indeed.  Thanks,

-- 
Peter Xu