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Diffstat (limited to 'results/classifier/118/all/42974450')
| -rw-r--r-- | results/classifier/118/all/42974450 | 454 |
1 files changed, 454 insertions, 0 deletions
diff --git a/results/classifier/118/all/42974450 b/results/classifier/118/all/42974450 new file mode 100644 index 000000000..a77e747b0 --- /dev/null +++ b/results/classifier/118/all/42974450 @@ -0,0 +1,454 @@ +debug: 0.924 +device: 0.921 +permissions: 0.918 +semantic: 0.917 +assembly: 0.917 +register: 0.916 +performance: 0.914 +user-level: 0.913 +PID: 0.913 +architecture: 0.912 +boot: 0.909 +network: 0.907 +graphic: 0.906 +arm: 0.901 +KVM: 0.901 +socket: 0.899 +risc-v: 0.896 +kernel: 0.895 +virtual: 0.893 +hypervisor: 0.889 +TCG: 0.887 +mistranslation: 0.882 +files: 0.877 +vnc: 0.869 +ppc: 0.864 +VMM: 0.853 +peripherals: 0.836 +x86: 0.814 +i386: 0.733 + +[Bug Report] Possible Missing Endianness Conversion + +The virtio packed virtqueue support patch[1] suggests converting +endianness by lines: + +virtio_tswap16s(vdev, &e->off_wrap); +virtio_tswap16s(vdev, &e->flags); + +Though both of these conversion statements aren't present in the +latest qemu code here[2] + +Is this intentional? + +[1]: +https://mail.gnu.org/archive/html/qemu-block/2019-10/msg01492.html +[2]: +https://elixir.bootlin.com/qemu/latest/source/hw/virtio/virtio.c#L314 + +CCing Jason. + +On Mon, Jun 24, 2024 at 4:30â¯PM Xoykie <xoykie@gmail.com> wrote: +> +> +The virtio packed virtqueue support patch[1] suggests converting +> +endianness by lines: +> +> +virtio_tswap16s(vdev, &e->off_wrap); +> +virtio_tswap16s(vdev, &e->flags); +> +> +Though both of these conversion statements aren't present in the +> +latest qemu code here[2] +> +> +Is this intentional? +Good catch! + +It looks like it was removed (maybe by mistake) by commit +d152cdd6f6 ("virtio: use virtio accessor to access packed event") + +Jason can you confirm that? + +Thanks, +Stefano + +> +> +[1]: +https://mail.gnu.org/archive/html/qemu-block/2019-10/msg01492.html +> +[2]: +https://elixir.bootlin.com/qemu/latest/source/hw/virtio/virtio.c#L314 +> + +On Mon, 24 Jun 2024 at 16:11, Stefano Garzarella <sgarzare@redhat.com> wrote: +> +> +CCing Jason. +> +> +On Mon, Jun 24, 2024 at 4:30â¯PM Xoykie <xoykie@gmail.com> wrote: +> +> +> +> The virtio packed virtqueue support patch[1] suggests converting +> +> endianness by lines: +> +> +> +> virtio_tswap16s(vdev, &e->off_wrap); +> +> virtio_tswap16s(vdev, &e->flags); +> +> +> +> Though both of these conversion statements aren't present in the +> +> latest qemu code here[2] +> +> +> +> Is this intentional? +> +> +Good catch! +> +> +It looks like it was removed (maybe by mistake) by commit +> +d152cdd6f6 ("virtio: use virtio accessor to access packed event") +That commit changes from: + +- address_space_read_cached(cache, off_off, &e->off_wrap, +- sizeof(e->off_wrap)); +- virtio_tswap16s(vdev, &e->off_wrap); + +which does a byte read of 2 bytes and then swaps the bytes +depending on the host endianness and the value of +virtio_access_is_big_endian() + +to this: + ++ e->off_wrap = virtio_lduw_phys_cached(vdev, cache, off_off); + +virtio_lduw_phys_cached() is a small function which calls +either lduw_be_phys_cached() or lduw_le_phys_cached() +depending on the value of virtio_access_is_big_endian(). +(And lduw_be_phys_cached() and lduw_le_phys_cached() do +the right thing for the host-endianness to do a "load +a specifically big or little endian 16-bit value".) + +Which is to say that because we use a load/store function that's +explicit about the size of the data type it is accessing, the +function itself can handle doing the load as big or little +endian, rather than the calling code having to do a manual swap after +it has done a load-as-bag-of-bytes. This is generally preferable +as it's less error-prone. + +(Explicit swap-after-loading still has a place where the +code is doing a load of a whole structure out of the +guest and then swapping each struct field after the fact, +because it means we can do a single load-from-guest-memory +rather than a whole sequence of calls all the way down +through the memory subsystem.) + +thanks +-- PMM + +On Mon, Jun 24, 2024 at 04:19:52PM GMT, Peter Maydell wrote: +On Mon, 24 Jun 2024 at 16:11, Stefano Garzarella <sgarzare@redhat.com> wrote: +CCing Jason. + +On Mon, Jun 24, 2024 at 4:30â¯PM Xoykie <xoykie@gmail.com> wrote: +> +> The virtio packed virtqueue support patch[1] suggests converting +> endianness by lines: +> +> virtio_tswap16s(vdev, &e->off_wrap); +> virtio_tswap16s(vdev, &e->flags); +> +> Though both of these conversion statements aren't present in the +> latest qemu code here[2] +> +> Is this intentional? + +Good catch! + +It looks like it was removed (maybe by mistake) by commit +d152cdd6f6 ("virtio: use virtio accessor to access packed event") +That commit changes from: + +- address_space_read_cached(cache, off_off, &e->off_wrap, +- sizeof(e->off_wrap)); +- virtio_tswap16s(vdev, &e->off_wrap); + +which does a byte read of 2 bytes and then swaps the bytes +depending on the host endianness and the value of +virtio_access_is_big_endian() + +to this: + ++ e->off_wrap = virtio_lduw_phys_cached(vdev, cache, off_off); + +virtio_lduw_phys_cached() is a small function which calls +either lduw_be_phys_cached() or lduw_le_phys_cached() +depending on the value of virtio_access_is_big_endian(). +(And lduw_be_phys_cached() and lduw_le_phys_cached() do +the right thing for the host-endianness to do a "load +a specifically big or little endian 16-bit value".) + +Which is to say that because we use a load/store function that's +explicit about the size of the data type it is accessing, the +function itself can handle doing the load as big or little +endian, rather than the calling code having to do a manual swap after +it has done a load-as-bag-of-bytes. This is generally preferable +as it's less error-prone. +Thanks for the details! + +So, should we also remove `virtio_tswap16s(vdev, &e->flags);` ? + +I mean: +diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c +index 893a072c9d..2e5e67bdb9 100644 +--- a/hw/virtio/virtio.c ++++ b/hw/virtio/virtio.c +@@ -323,7 +323,6 @@ static void vring_packed_event_read(VirtIODevice *vdev, + /* Make sure flags is seen before off_wrap */ + smp_rmb(); + e->off_wrap = virtio_lduw_phys_cached(vdev, cache, off_off); +- virtio_tswap16s(vdev, &e->flags); + } + + static void vring_packed_off_wrap_write(VirtIODevice *vdev, + +Thanks, +Stefano +(Explicit swap-after-loading still has a place where the +code is doing a load of a whole structure out of the +guest and then swapping each struct field after the fact, +because it means we can do a single load-from-guest-memory +rather than a whole sequence of calls all the way down +through the memory subsystem.) + +thanks +-- PMM + +On Tue, 25 Jun 2024 at 08:18, Stefano Garzarella <sgarzare@redhat.com> wrote: +> +> +On Mon, Jun 24, 2024 at 04:19:52PM GMT, Peter Maydell wrote: +> +>On Mon, 24 Jun 2024 at 16:11, Stefano Garzarella <sgarzare@redhat.com> wrote: +> +>> +> +>> CCing Jason. +> +>> +> +>> On Mon, Jun 24, 2024 at 4:30â¯PM Xoykie <xoykie@gmail.com> wrote: +> +>> > +> +>> > The virtio packed virtqueue support patch[1] suggests converting +> +>> > endianness by lines: +> +>> > +> +>> > virtio_tswap16s(vdev, &e->off_wrap); +> +>> > virtio_tswap16s(vdev, &e->flags); +> +>> > +> +>> > Though both of these conversion statements aren't present in the +> +>> > latest qemu code here[2] +> +>> > +> +>> > Is this intentional? +> +>> +> +>> Good catch! +> +>> +> +>> It looks like it was removed (maybe by mistake) by commit +> +>> d152cdd6f6 ("virtio: use virtio accessor to access packed event") +> +> +> +>That commit changes from: +> +> +> +>- address_space_read_cached(cache, off_off, &e->off_wrap, +> +>- sizeof(e->off_wrap)); +> +>- virtio_tswap16s(vdev, &e->off_wrap); +> +> +> +>which does a byte read of 2 bytes and then swaps the bytes +> +>depending on the host endianness and the value of +> +>virtio_access_is_big_endian() +> +> +> +>to this: +> +> +> +>+ e->off_wrap = virtio_lduw_phys_cached(vdev, cache, off_off); +> +> +> +>virtio_lduw_phys_cached() is a small function which calls +> +>either lduw_be_phys_cached() or lduw_le_phys_cached() +> +>depending on the value of virtio_access_is_big_endian(). +> +>(And lduw_be_phys_cached() and lduw_le_phys_cached() do +> +>the right thing for the host-endianness to do a "load +> +>a specifically big or little endian 16-bit value".) +> +> +> +>Which is to say that because we use a load/store function that's +> +>explicit about the size of the data type it is accessing, the +> +>function itself can handle doing the load as big or little +> +>endian, rather than the calling code having to do a manual swap after +> +>it has done a load-as-bag-of-bytes. This is generally preferable +> +>as it's less error-prone. +> +> +Thanks for the details! +> +> +So, should we also remove `virtio_tswap16s(vdev, &e->flags);` ? +> +> +I mean: +> +diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c +> +index 893a072c9d..2e5e67bdb9 100644 +> +--- a/hw/virtio/virtio.c +> ++++ b/hw/virtio/virtio.c +> +@@ -323,7 +323,6 @@ static void vring_packed_event_read(VirtIODevice *vdev, +> +/* Make sure flags is seen before off_wrap */ +> +smp_rmb(); +> +e->off_wrap = virtio_lduw_phys_cached(vdev, cache, off_off); +> +- virtio_tswap16s(vdev, &e->flags); +> +} +That definitely looks like it's probably not correct... + +-- PMM + +On Fri, Jun 28, 2024 at 03:53:09PM GMT, Peter Maydell wrote: +On Tue, 25 Jun 2024 at 08:18, Stefano Garzarella <sgarzare@redhat.com> wrote: +On Mon, Jun 24, 2024 at 04:19:52PM GMT, Peter Maydell wrote: +>On Mon, 24 Jun 2024 at 16:11, Stefano Garzarella <sgarzare@redhat.com> wrote: +>> +>> CCing Jason. +>> +>> On Mon, Jun 24, 2024 at 4:30â¯PM Xoykie <xoykie@gmail.com> wrote: +>> > +>> > The virtio packed virtqueue support patch[1] suggests converting +>> > endianness by lines: +>> > +>> > virtio_tswap16s(vdev, &e->off_wrap); +>> > virtio_tswap16s(vdev, &e->flags); +>> > +>> > Though both of these conversion statements aren't present in the +>> > latest qemu code here[2] +>> > +>> > Is this intentional? +>> +>> Good catch! +>> +>> It looks like it was removed (maybe by mistake) by commit +>> d152cdd6f6 ("virtio: use virtio accessor to access packed event") +> +>That commit changes from: +> +>- address_space_read_cached(cache, off_off, &e->off_wrap, +>- sizeof(e->off_wrap)); +>- virtio_tswap16s(vdev, &e->off_wrap); +> +>which does a byte read of 2 bytes and then swaps the bytes +>depending on the host endianness and the value of +>virtio_access_is_big_endian() +> +>to this: +> +>+ e->off_wrap = virtio_lduw_phys_cached(vdev, cache, off_off); +> +>virtio_lduw_phys_cached() is a small function which calls +>either lduw_be_phys_cached() or lduw_le_phys_cached() +>depending on the value of virtio_access_is_big_endian(). +>(And lduw_be_phys_cached() and lduw_le_phys_cached() do +>the right thing for the host-endianness to do a "load +>a specifically big or little endian 16-bit value".) +> +>Which is to say that because we use a load/store function that's +>explicit about the size of the data type it is accessing, the +>function itself can handle doing the load as big or little +>endian, rather than the calling code having to do a manual swap after +>it has done a load-as-bag-of-bytes. This is generally preferable +>as it's less error-prone. + +Thanks for the details! + +So, should we also remove `virtio_tswap16s(vdev, &e->flags);` ? + +I mean: +diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c +index 893a072c9d..2e5e67bdb9 100644 +--- a/hw/virtio/virtio.c ++++ b/hw/virtio/virtio.c +@@ -323,7 +323,6 @@ static void vring_packed_event_read(VirtIODevice *vdev, + /* Make sure flags is seen before off_wrap */ + smp_rmb(); + e->off_wrap = virtio_lduw_phys_cached(vdev, cache, off_off); +- virtio_tswap16s(vdev, &e->flags); + } +That definitely looks like it's probably not correct... +Yeah, I just sent that patch: +20240701075208.19634-1-sgarzare@redhat.com +">https://lore.kernel.org/qemu-devel/ +20240701075208.19634-1-sgarzare@redhat.com +We can continue the discussion there. + +Thanks, +Stefano + |