Re: [PATCH 3/3] util/userfaultfd: Support /dev/userfaultfd

[Peter Xu]

On Thu, Jan 26, 2023 at 12:08:33AM +0100, Philippe Mathieu-Daudé wrote:
> On 25/1/23 23:40, Peter Xu wrote:
> > Teach QEMU to use /dev/userfaultfd when it existed and fallback to the
> > system call if either it's not there or doesn't have enough permission.
> > 
> > Firstly, as long as the app has permission to access /dev/userfaultfd, it
> > always have the ability to trap kernel faults which QEMU mostly wants.
> > Meanwhile, in some context (e.g. containers) the userfaultfd syscall can be
> > forbidden, so it can be the major way to use postcopy in a restricted
> > environment with strict seccomp setup.
> > 
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >   util/trace-events  |  1 +
> >   util/userfaultfd.c | 36 ++++++++++++++++++++++++++++++++++++
> >   2 files changed, 37 insertions(+)
> > 
> > diff --git a/util/trace-events b/util/trace-events
> > index c8f53d7d9f..16f78d8fe5 100644
> > --- a/util/trace-events
> > +++ b/util/trace-events
> > @@ -93,6 +93,7 @@ qemu_vfio_region_info(const char *desc, uint64_t 
> > region_ofs, uint64_t region_siz
> >   qemu_vfio_pci_map_bar(int index, uint64_t region_ofs, uint64_t 
> > region_size, int ofs, void *host) "map region bar#%d addr 0x%"PRIx64" size 
> > 0x%"PRIx64" ofs 0x%x host %p"
> >   #userfaultfd.c
> > +uffd_detect_open_mode(int mode) "%d"
> >   uffd_query_features_nosys(int err) "errno: %i"
> >   uffd_query_features_api_failed(int err) "errno: %i"
> >   uffd_create_fd_nosys(int err) "errno: %i"
> > diff --git a/util/userfaultfd.c b/util/userfaultfd.c
> > index 9845a2ec81..360ecf8084 100644
> > --- a/util/userfaultfd.c
> > +++ b/util/userfaultfd.c
> > @@ -18,10 +18,46 @@
> >   #include <poll.h>
> >   #include <sys/syscall.h>
> >   #include <sys/ioctl.h>
> > +#include <fcntl.h>
> > +
> > +typedef enum {
> > +    UFFD_UNINITIALIZED = 0,
> > +    UFFD_USE_DEV_PATH,
> > +    UFFD_USE_SYSCALL,
> > +} uffd_open_mode;
> > +
> > +static uffd_open_mode open_mode;
> 
> 'open_mode' could be reduced to uffd_detect_open_mode()'s
> scope.

Yes, will do.

> 
> > +static int uffd_dev;
> > +
> > +static uffd_open_mode uffd_detect_open_mode(void)
> > +{
> > +    if (open_mode == UFFD_UNINITIALIZED) {
> > +        /*
> > +         * Make /dev/userfaultfd the default approach because it has better
> > +         * permission controls, meanwhile allows kernel faults without any
> > +         * privilege requirement (e.g. SYS_CAP_PTRACE).
> > +         */
> > +        uffd_dev = open("/dev/userfaultfd", O_RDWR | O_CLOEXEC);
> > +        if (uffd_dev >= 0) {
> > +            open_mode = UFFD_USE_DEV_PATH;
> > +        } else {
> > +            /* Fallback to the system call */
> > +            open_mode = UFFD_USE_SYSCALL;
> > +        }
> > +        trace_uffd_detect_open_mode(open_mode);
> > +    }
> > +
> > +    return open_mode;
> 
> If 'open_mode' isn't relevant, this function could return uffd_dev/-1
> instead. Not really an improvement :)

Logically I think the two variables can be squashed into one.  I kept that
for clearance just to easily identify e.g. uffd_dev is not chosen to be
used, or uffd_dev open failed.

> 
> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

Thanks, Phil.

-- 
Peter Xu