linux-kernel - Re: [Question-kvm] Can hva_to_pfn_fast be executed in interrupt context?

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Date:   Thu, 15 Aug 2019 22:48:35 +0530
From:   Bharath Vedartham <linux.bhar@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     rkrcmar@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        khalid.aziz@...cle.com
Subject: Re: [Question-kvm] Can hva_to_pfn_fast be executed in interrupt
 context?

On Tue, Aug 13, 2019 at 10:17:09PM +0200, Paolo Bonzini wrote:
> On 13/08/19 21:14, Bharath Vedartham wrote:
> > Hi all,
> > 
> > I was looking at the function hva_to_pfn_fast(in virt/kvm/kvm_main) which is 
> > executed in an atomic context(even in non-atomic context, since
> > hva_to_pfn_fast is much faster than hva_to_pfn_slow).
> > 
> > My question is can this be executed in an interrupt context? 
> 
> No, it cannot for the reason you mention below.
> 
> Paolo
hmm.. Well I expected the answer to be kvm specific. 
Because I observed a similar use-case for a driver (sgi-gru) where 
we want to retrive the physical address of a virtual address. This was
done in atomic and non-atomic context similar to hva_to_pfn_fast and
hva_to_pfn_slow. __get_user_pages_fast(for atomic case) 
would not work as the driver could execute in interrupt context.

The driver manually walked the page tables to handle this issue.

Since kvm is a widely used piece of code, I asked this question to know
how kvm handled this issue. 

Thank you for your time.

Thank you
Bharath
> > The motivation for this question is that in an interrupt context, we cannot
> > assume "current" to be the task_struct of the process of interest.
> > __get_user_pages_fast assume current->mm when walking the process page
> > tables. 
> > 
> > So if this function hva_to_pfn_fast can be executed in an
> > interrupt context, it would not be safe to retrive the pfn with
> > __get_user_pages_fast. 
> > 
> > Thoughts on this?
> > 
> > Thank you
> > Bharath
> > 
>