linux-kernel - Re: [PATCH] vfio/pci: take mmap write lock for io_remap_pfn

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Message-ID: <e7849fb1-e333-6210-6c92-b00ecb52ccd4@redhat.com>
Date:   Thu, 11 May 2023 09:32:06 +0200
From:   Cédric Le Goater <clg@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Alex Williamson <alex.williamson@...hat.com>
Cc:     Yan Zhao <yan.y.zhao@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, kevin.tian@...el.com,
        yishaih@...dia.com, shameerali.kolothum.thodi@...wei.com
Subject: Re: [PATCH] vfio/pci: take mmap write lock for io_remap_pfn_range

On 5/10/23 22:41, Jason Gunthorpe wrote:
> On Mon, May 08, 2023 at 02:57:15PM -0600, Alex Williamson wrote:
> 
>> We already try to set the flags in advance, but there are some
>> architectural flags like VM_PAT that make that tricky.  Cedric has been
>> looking at inserting individual pages with vmf_insert_pfn(), but that
>> incurs a lot more faults and therefore latency vs remapping the entire
>> vma on fault.  I'm not convinced that we shouldn't just attempt to
>> remove the fault handler entirely, but I haven't tried it yet to know
>> what gotchas are down that path.  Thanks,

OTOH I didn't see any noticeable slowdowns in the tests I did with
NICs, IGD and NVIDIA GPU pass-through. I lack devices with large
BARs though. If anyone has some time for it, here are commits :

   https://github.com/legoater/linux/commits/vfio

First does a one page insert and fixes the lockdep issues we are
seeing with the recent series:

   https://lore.kernel.org/all/20230126193752.297968-1-surenb@google.com/

Second adds some statistics. For the NVIDIA GPU, faults reach 800k.

Thanks,

C.

> I thought we did it like this because there were races otherwise with
> PTE insertion and zapping? I don't remember well anymore.
> 
> I vaugely remember the address_space conversion might help remove the
> fault handler?
> 
> Jason
>