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Message-ID: <20190820160821.GA5153@bharath12345-Inspiron-5559> References: <20190813191435.GB10228@bharath12345-Inspiron-5559> <54182261-88a4-9970-1c3c-8402e130dcda@redhat.com> <20190815171834.GA14342@bharath12345-Inspiron-5559> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 > > > > > > > > >