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From: Peter Xu <peterx@redhat.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Alex Williamson <alex.williamson@redhat.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	cohuck@redhat.com
Subject: Re: [PATCH v2 2/3] vfio-pci: Fault mmaps to enable vma tracking
Date: Thu, 7 May 2020 22:16:56 -0400
Message-ID: <20200508021656.GS228260@xz-x1> (raw)
In-Reply-To: <20200507235633.GL26002@ziepe.ca>

On Thu, May 07, 2020 at 08:56:33PM -0300, Jason Gunthorpe wrote:
> On Thu, May 07, 2020 at 06:22:23PM -0400, Peter Xu wrote:
> > On Thu, May 07, 2020 at 04:03:34PM -0600, Alex Williamson wrote:
> > > On Thu, 7 May 2020 17:47:44 -0400
> > > Peter Xu <peterx@redhat.com> wrote:
> > > 
> > > > Hi, Alex,
> > > > 
> > > > On Tue, May 05, 2020 at 03:54:53PM -0600, Alex Williamson wrote:
> > > > > +/*
> > > > > + * Zap mmaps on open so that we can fault them in on access and therefore
> > > > > + * our vma_list only tracks mappings accessed since last zap.
> > > > > + */
> > > > > +static void vfio_pci_mmap_open(struct vm_area_struct *vma)
> > > > > +{
> > > > > +	zap_vma_ptes(vma, vma->vm_start, vma->vm_end - vma->vm_start);  
> > > > 
> > > > A pure question: is this only a safety-belt or it is required in some known
> > > > scenarios?
> > > 
> > > It's not required.  I originally did this so that I'm not allocating a
> > > vma_list entry in a path where I can't return error, but as Jason
> > > suggested I could zap here only in the case that I do encounter that
> > > allocation fault.  However I still like consolidating the vma_list
> > > handling to the vm_ops .fault and .close callbacks and potentially we
> > > reduce the zap latency by keeping the vma_list to actual users, which
> > > we'll get to eventually anyway in the VM case as memory BARs are sized
> > > and assigned addresses.
> > 
> > Yes, I don't see much problem either on doing the vma_list maintainance only in
> > .fault() and .close().  My understandingg is that the worst case is the perf
> > critical applications (e.g. DPDK) could pre-fault these MMIO region easily
> > during setup if they want.  My question was majorly about whether the vma
> > should be guaranteed to have no mapping at all when .open() is called.  But I
> > agree with you that it's always good to have that as safety-belt anyways.
> 
> If the VMA has a mapping then that specific VMA has to be in the
> linked list.
> 
> So if the zap is skipped then the you have to allocate something and
> add to the linked list to track the VMA with mapping.
> 
> It is not a 'safety belt'

But shouldn't open() only be called when the VMA is created for a memory range?
If so, does it also mean that the address range must have not been mapped yet?

Thanks,

-- 
Peter Xu


  reply index

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-05 21:54 [PATCH v2 0/3] vfio-pci: Block user access to disabled device MMIO Alex Williamson
2020-05-05 21:54 ` [PATCH v2 1/3] vfio/type1: Support faulting PFNMAP vmas Alex Williamson
2020-05-07 21:24   ` Peter Xu
2020-05-07 21:47     ` Alex Williamson
2020-05-07 23:54     ` Jason Gunthorpe
2020-05-08  2:19       ` Peter Xu
2020-05-08 12:10         ` Jason Gunthorpe
2020-05-08 14:30           ` Peter Xu
2020-05-08 15:05             ` Jason Gunthorpe
2020-05-08 15:42               ` Alex Williamson
2020-05-08 16:05                 ` Peter Xu
2020-05-08 18:39   ` Peter Xu
2020-05-05 21:54 ` [PATCH v2 2/3] vfio-pci: Fault mmaps to enable vma tracking Alex Williamson
2020-05-07 21:47   ` Peter Xu
2020-05-07 22:03     ` Alex Williamson
2020-05-07 22:22       ` Peter Xu
2020-05-07 23:56         ` Jason Gunthorpe
2020-05-08  2:16           ` Peter Xu [this message]
2020-05-08  6:44             ` Jason Wang
2020-05-08 14:27               ` Peter Xu
2020-05-08 12:08             ` Jason Gunthorpe
2020-05-08 14:26               ` Peter Xu
2020-05-05 21:55 ` [PATCH v2 3/3] vfio-pci: Invalidate mmaps and block MMIO access on disabled memory Alex Williamson
2020-05-22  2:39   ` Qian Cai
2020-05-22  4:18     ` Alex Williamson
2020-05-07 21:59 ` [PATCH v2 0/3] vfio-pci: Block user access to disabled device MMIO Peter Xu
2020-05-07 22:34   ` Alex Williamson
2020-05-08  2:31     ` Peter Xu

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