Re: [PATCH v3 3/3] vfio: selftests: Test BAR write faults after clear_refs

[Mina Almasry]

On Tue, Nov 3, 2026 at 9:27 AM Alex Williamson <alex@xxxxxxxxxxx> wrote:
>
> On Mon,  2 Nov 2026 23:04:45 +0000
> Mina Almasry <almasrymina@xxxxxxxxxx> wrote:
>
> > Add a regression test in vfio_pci_device_test to verify that huge_fault
> > on writable MAP_SHARED VFIO PCI BAR mappings handles write faults after
> > soft-dirty tracking clears VM_SOFTDIRTY and downgrades vma->vm_page_prot
> > to read-only via /proc/self/clear_refs ("4").
> >
> > Use vfio_pci_find_mem_bar() to locate a memory-backed BAR of at least
> > 2 MiB that is safe to read and write, and zap existing BAR mappings via
> > PCI_COMMAND_MEMORY before each sub-test to verify both:
> >
> > 1. Read-then-write fault sequence after clear_refs, exercising in-place
> >    huge PMD/PUD write-protection upgrade via wp_huge_pmd()/wp_huge_pud().
> > 2. Direct initial write fault on an unpopulated BAR VMA after clear_refs,
> >    exercising vmf_insert_pfn_pmd()/vmf_insert_pfn_pud() with
> >    FAULT_FLAG_WRITE set.
> >
> > Skip gracefully when the device has no known memory-backed BAR >= 2 MiB
> > or when /proc/self/clear_refs soft-dirty tracking (CONFIG_MEM_SOFT_DIRTY)
> > is unavailable.
> >
> > Suggested-by: Alex Williamson <alex@xxxxxxxxxxx>
> > Cc: David Matlack <dmatlack@xxxxxxxxxx>
> > Cc: Matt Evans <matt@xxxxxxxxxx>
> > Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>
> > ---
> >  .../selftests/vfio/vfio_pci_device_test.c     | 65 +++++++++++++++++++
> >  1 file changed, 65 insertions(+)
> >
> > diff --git a/tools/testing/selftests/vfio/vfio_pci_device_test.c b/tools/testing/selftests/vfio/vfio_pci_device_test.c
> > index 93c11fd5e081..323a316f59e7 100644
> > --- a/tools/testing/selftests/vfio/vfio_pci_device_test.c
> > +++ b/tools/testing/selftests/vfio/vfio_pci_device_test.c
> > @@ -1,10 +1,13 @@
> >  // SPDX-License-Identifier: GPL-2.0-only
> > +#include <errno.h>
> >  #include <fcntl.h>
> >  #include <stdlib.h>
> > +#include <unistd.h>
> >
> >  #include <sys/ioctl.h>
> >  #include <sys/mman.h>
> >
> > +#include <linux/compiler.h>
> >  #include <linux/limits.h>
> >  #include <linux/pci_regs.h>
> >  #include <linux/sizes.h>
> > @@ -102,6 +105,68 @@ TEST_F(vfio_pci_device_test, validate_bars)
> >       }
> >  }
> >
> > +static void zap_bar_mappings(struct vfio_pci_device *device)
> > +{
> > +     vfio_pci_cmd_clear(device, PCI_COMMAND_MEMORY);
> > +     vfio_pci_cmd_set(device, PCI_COMMAND_MEMORY);
> > +}
> > +
> > +static int clear_soft_dirty_refs(void)
> > +{
> > +     ssize_t ret;
> > +     int fd;
> > +
> > +     fd = open("/proc/self/clear_refs", O_WRONLY);
> > +     if (fd < 0)
> > +             return -errno;
> > +
> > +     ret = write(fd, "4\n", 2);
> > +     if (ret < 0)
> > +             ret = -errno;
> > +     else
> > +             VFIO_ASSERT_EQ(ret, 2);
> > +
> > +     VFIO_ASSERT_EQ(close(fd), 0);
> > +     return ret < 0 ? ret : 0;
> > +}
> > +
> > +TEST_F(vfio_pci_device_test, bar_write_fault_after_clear_refs)
> > +{
> > +     struct vfio_pci_mem_bar mem_bar;
> > +     int ret;
> > +     u32 val;
> > +
> > +     if (!vfio_pci_find_mem_bar(self->device, SZ_2M, &mem_bar))
> > +             SKIP(return, "No memory-backed BAR >= 2M on %s\n", device_bdf);
> > +
> > +     zap_bar_mappings(self->device);
> > +
> > +     ret = clear_soft_dirty_refs();
> > +     if (ret == -ENOENT || ret == -EINVAL)
> > +             SKIP(return, "soft-dirty clear_refs unsupported (%d)\n", ret);
>
> My comment on v2 that this doesn't accurately test whether soft-dirty
> is supported, potentially leading to a false pass still stands.
>
> Please also have the courtesy to give reasonable time for review before
> a respin.  Personally I would expect self review and testing to take
> longer than the 80 minutes between David's feedback and v3.  Thanks,
>

Ack, my apologies. Will do.

-- 
Thanks,
Mina