linux-kernel - Re: [PATCH] vfio-pci: Avoid recursive read-lock usage

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Message-ID: <20200806113645.47de0bfa.cohuck@redhat.com>
Date:   Thu, 6 Aug 2020 11:36:45 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfio-pci: Avoid recursive read-lock usage

On Wed, 05 Aug 2020 11:58:05 -0600
Alex Williamson <alex.williamson@...hat.com> wrote:

> A down_read on memory_lock is held when performing read/write accesses
> to MMIO BAR space, including across the copy_to/from_user() callouts
> which may fault.  If the user buffer for these copies resides in an
> mmap of device MMIO space, the mmap fault handler will acquire a
> recursive read-lock on memory_lock.  Avoid this by reducing the lock
> granularity.  Sequential accesses requiring multiple ioread/iowrite
> cycles are expected to be rare, therefore typical accesses should not
> see additional overhead.
> 
> VGA MMIO accesses are expected to be non-fatal regardless of the PCI
> memory enable bit to allow legacy probing, this behavior remains with
> a comment added.  ioeventfds are now included in memory access testing,
> with writes dropped while memory space is disabled.
> 
> Fixes: abafbc551fdd ("vfio-pci: Invalidate mmaps and block MMIO access on disabled memory")
> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> ---
>  drivers/vfio/pci/vfio_pci_private.h |    2 +
>  drivers/vfio/pci/vfio_pci_rdwr.c    |  120 ++++++++++++++++++++++++++++-------
>  2 files changed, 98 insertions(+), 24 deletions(-)

Reviewed-by: Cornelia Huck <cohuck@...hat.com>