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

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Date:   Thu, 23 Jun 2022 14:31:47 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Peter Xu <peterx@...hat.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 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.

> +}
> +
>  /*
>   * error_noslot pfns indicate that the gfn can not be
>   * translated to pfn - it is not in slot or failed to
> -- 
> 2.32.0
>