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