* [Question-kvm] Can hva_to_pfn_fast be executed in interrupt context?
@ 2019-08-13 19:14 Bharath Vedartham
2019-08-13 20:17 ` Paolo Bonzini
0 siblings, 1 reply; 4+ messages in thread
From: Bharath Vedartham @ 2019-08-13 19:14 UTC (permalink / raw)
To: pbonzini, rkrcmar; +Cc: kvm, linux-kernel, linux-mm, khalid.aziz
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?
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
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [Question-kvm] Can hva_to_pfn_fast be executed in interrupt context?
2019-08-13 19:14 [Question-kvm] Can hva_to_pfn_fast be executed in interrupt context? Bharath Vedartham
@ 2019-08-13 20:17 ` Paolo Bonzini
2019-08-15 17:18 ` Bharath Vedartham
0 siblings, 1 reply; 4+ messages in thread
From: Paolo Bonzini @ 2019-08-13 20:17 UTC (permalink / raw)
To: Bharath Vedartham, rkrcmar; +Cc: kvm, linux-kernel, linux-mm, khalid.aziz
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
> 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
>
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [Question-kvm] Can hva_to_pfn_fast be executed in interrupt context?
2019-08-13 20:17 ` Paolo Bonzini
@ 2019-08-15 17:18 ` Bharath Vedartham
[not found] ` <CABgObfbQOS28cG_9Ca_2OXbLmDy_hwUkuqPnzJG5=FZ5sEYGfA@mail.gmail.com>
0 siblings, 1 reply; 4+ messages in thread
From: Bharath Vedartham @ 2019-08-15 17:18 UTC (permalink / raw)
To: Paolo Bonzini; +Cc: rkrcmar, kvm, linux-kernel, linux-mm, khalid.aziz
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
> >
>
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [Question-kvm] Can hva_to_pfn_fast be executed in interrupt context?
[not found] ` <CABgObfbQOS28cG_9Ca_2OXbLmDy_hwUkuqPnzJG5=FZ5sEYGfA@mail.gmail.com>
@ 2019-08-20 16:08 ` Bharath Vedartham
0 siblings, 0 replies; 4+ messages in thread
From: Bharath Vedartham @ 2019-08-20 16:08 UTC (permalink / raw)
To: Paolo Bonzini; +Cc: Radim Krcmar, kvm, linux-kernel, linux-mm, khalid.aziz
On Thu, Aug 15, 2019 at 08:26:43PM +0200, Paolo Bonzini wrote:
> Oh, I see. Sorry I didn't understand the question. In the case of KVM,
> there's simply no code that runs in interrupt context and needs to use
> virtual addresses.
>
> In fact, there's no code that runs in interrupt context at all. The only
> code that deals with host interrupts in a virtualization host is in VFIO,
> but all it needs to do is signal an eventfd.
>
> Paolo
Great, answers my question. Thank you for your time.
Thank you
Bharath
>
> Il gio 15 ago 2019, 19:18 Bharath Vedartham <linux.bhar@gmail.com> ha
> scritto:
>
> > 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
> > > >
> > >
> >
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2019-08-20 16:08 UTC | newest]
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2019-08-13 19:14 [Question-kvm] Can hva_to_pfn_fast be executed in interrupt context? Bharath Vedartham
2019-08-13 20:17 ` Paolo Bonzini
2019-08-15 17:18 ` Bharath Vedartham
[not found] ` <CABgObfbQOS28cG_9Ca_2OXbLmDy_hwUkuqPnzJG5=FZ5sEYGfA@mail.gmail.com>
2019-08-20 16:08 ` Bharath Vedartham
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