Re: [PATCH] vfio/pci: make the vfio_pci_mmap_fault reentrant

From: Alex Williamson <alex.williamson@redhat.com>
To: Peter Xu <peterx@redhat.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>,
	"Zengtao (B)" <prime.zeng@hisilicon.com>,
	Cornelia Huck <cohuck@redhat.com>,
	Kevin Tian <kevin.tian@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Giovanni Cabiddu <giovanni.cabiddu@intel.com>,
	Michel Lespinasse <walken@google.com>,
	Jann Horn <jannh@google.com>, Max Gurtovoy <mgurtovoy@nvidia.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Linuxarm <linuxarm@huawei.com>
Subject: Re: [PATCH] vfio/pci: make the vfio_pci_mmap_fault reentrant
Date: Tue, 9 Mar 2021 14:43:01 -0700	[thread overview]
Message-ID: <20210309144301.3593af00@omen.home.shazbot.org> (raw)
In-Reply-To: <20210309210036.GJ763132@xz-x1>

On Tue, 9 Mar 2021 16:00:36 -0500
Peter Xu <peterx@redhat.com> wrote:

> On Tue, Mar 09, 2021 at 01:11:04PM -0700, Alex Williamson wrote:
> > > It's just that the initial MMIO access delay would be spread to the 1st access
> > > of each mmio page access rather than using the previous pre-fault scheme.  I
> > > think an userspace cares the delay enough should pre-fault all pages anyway,
> > > but just raise this up.  Otherwise looks sane.  
> > 
> > Yep, this is a concern.  Is it safe to have loops concurrently and fully
> > populating the same vma with vmf_insert_pfn()?  
> 
> AFAIU it's safe, and probably the (so far) best way for an userspace to quickly
> populate a huge chunk of mmap()ed region for either MMIO or RAM.  Indeed from
> that pov vmf_insert_pfn() seems to be even more efficient on prefaulting since
> it can be threaded.

Ok, then we'll keep the loop and expect that a race might incur
duplicate work, but should be safe.

It also occurred to me that Jason was suggesting the _prot version of
vmf_insert_pfn(), which I think is necessary if we want to keep the
same semantics where the default io_remap_pfn_range() was applying
pgprot_decrypted() onto vma->vm_page_prot.  So if we don't want to
break SME use cases we better apply that ourselves.  Thanks,

Alex