Re: [PATCH v2 1/3] hw/net: e1000: Correct the initial value of VET register

[Bin Meng <bmeng.cn@gmail.com>]

On Wed, Jul 14, 2021 at 4:40 PM Jason Wang <jasowang@redhat.com> wrote:
>
>
> 在 2021/7/14 下午2:04, Bin Meng 写道:
> > On Wed, Jul 14, 2021 at 12:53 PM Jason Wang <jasowang@redhat.com> wrote:
> >>
> >> 在 2021/7/14 上午11:42, Bin Meng 写道:
> >>> On Wed, Jul 14, 2021 at 11:10 AM Jason Wang <jasowang@redhat.com> wrote:
> >>>> 在 2021/7/13 下午5:11, Bin Meng 写道:
> >>>>> On Tue, Jul 13, 2021 at 5:02 PM Jason Wang <jasowang@redhat.com> wrote:
> >>>>>> 在 2021/7/13 下午4:36, Bin Meng 写道:
> >>>>>>> On Tue, Jul 13, 2021 at 3:03 PM Jason Wang <jasowang@redhat.com> wrote:
> >>>>>>>> 在 2021/7/13 上午7:06, Bin Meng 写道:
> >>>>>>>>> On Mon, Jul 5, 2021 at 1:57 PM Bin Meng <bmeng.cn@gmail.com> wrote:
> >>>>>>>>>> On Mon, Jul 5, 2021 at 12:21 PM Jason Wang <jasowang@redhat.com> wrote:
> >>>>>>>>>>> 在 2021/7/2 下午5:24, Bin Meng 写道:
> >>>>>>>>>>>> From: Christina Wang <christina.wang@windriver.com>
> >>>>>>>>>>>>
> >>>>>>>>>>>> The initial value of VLAN Ether Type (VET) register is 0x8100, as per
> >>>>>>>>>>>> the manual and real hardware.
> >>>>>>>>>>>>
> >>>>>>>>>>>> While Linux e1000 driver always writes VET register to 0x8100, it is
> >>>>>>>>>>>> not always the case for everyone. Drivers relying on the reset value
> >>>>>>>>>>>> of VET won't be able to transmit and receive VLAN frames in QEMU.
> >>>>>>>>>>>>
> >>>>>>>>>>>> Reported-by: Markus Carlstedt <markus.carlstedt@windriver.com>
> >>>>>>>>>>>> Signed-off-by: Christina Wang <christina.wang@windriver.com>
> >>>>>>>>>>>> Signed-off-by: Bin Meng <bin.meng@windriver.com>
> >>>>>>>>>>>> ---
> >>>>>>>>>>>>
> >>>>>>>>>>>> (no changes since v1)
> >>>>>>>>>>>>
> >>>>>>>>>>>>        hw/net/e1000.c | 2 ++
> >>>>>>>>>>>>        1 file changed, 2 insertions(+)
> >>>>>>>>>>>>
> >>>>>>>>>>>> diff --git a/hw/net/e1000.c b/hw/net/e1000.c
> >>>>>>>>>>>> index 4f75b44cfc..20cbba6411 100644
> >>>>>>>>>>>> --- a/hw/net/e1000.c
> >>>>>>>>>>>> +++ b/hw/net/e1000.c
> >>>>>>>>>>>> @@ -29,6 +29,7 @@
> >>>>>>>>>>>>        #include "hw/pci/pci.h"
> >>>>>>>>>>>>        #include "hw/qdev-properties.h"
> >>>>>>>>>>>>        #include "migration/vmstate.h"
> >>>>>>>>>>>> +#include "net/eth.h"
> >>>>>>>>>>>>        #include "net/net.h"
> >>>>>>>>>>>>        #include "net/checksum.h"
> >>>>>>>>>>>>        #include "sysemu/sysemu.h"
> >>>>>>>>>>>> @@ -254,6 +255,7 @@ static const uint32_t mac_reg_init[] = {
> >>>>>>>>>>>>            [MANC]    = E1000_MANC_EN_MNG2HOST | E1000_MANC_RCV_TCO_EN |
> >>>>>>>>>>>>                        E1000_MANC_ARP_EN | E1000_MANC_0298_EN |
> >>>>>>>>>>>>                        E1000_MANC_RMCP_EN,
> >>>>>>>>>>>> +    [VET]     = ETH_P_VLAN,
> >>>>>>>>>>> I wonder if we need a compat flag for this, since we change the behavior.
> >>>>>>>>>>>
> >>>>>>>>>>> (See e1000_properties[])
> >>>>>>>>>>>
> >>>>>>>>>> No we don't need to since it does not break migration.
> >>>>>>>>> Ping?
> >>>>>>>> I admit migration "works" but it doesn't mean it's not broken. It
> >>>>>>>> changes the guest visible default value of VET register, so it may break
> >>>>>>>> things silently for the guest.
> >>>>>>>>
> >>>>>>>> For old machine types, we should stick the value to the one without this
> >>>>>>>> fix.
> >>>>>>> Could you please propose a solution on how to handle such a scenario
> >>>>>>> in a generic way in QEMU? (+Peter)
> >>>>>> Well, I think I've suggested you to have a look at how things is done in
> >>>>>> for handling such compatibility in e1000_properties.
> >>>>>>
> >>>>>>
> >>>>>>> The POR reset value is wrong in QEMU and has carried forward the wrong
> >>>>>>> value for years, and correcting it to its right value needs to do
> >>>>>>> what?
> >>>>>> We should stick to the wrong behavior for old machine types.
> >>>>>>
> >>>>>> That's all.
> >>>>> So that means the following SD patch is also wrong (+Philippe) which
> >>>>> changes the default value of capability register.
> >>>>> http://patchwork.ozlabs.org/project/qemu-devel/patch/20210623185921.24113-1-joannekoong@gmail.com/
> >>>> It should compat capareg for the old value for old machine types.
> >>> Yeah, it's already a property for the SD controller model but someone
> >>> views it as a bug because the model implements 64-bit but not
> >>> reporting it in the capability register.
> >>>
> >>>>> Can we get some agreement among maintainers?
> >>>> It's not about the agreement but about to have a stable ABI. I don't
> >>>> know the case for sd but e1000 is used in various  and we work hard to
> >>>> unbreak the migration compatibility among downstream versions. Git log
> >>>> on e1000.c will tell you more.
> >>> Agreement or stable ABI, whatever we call, but we should be in some consistency.
> >>>
> >>> IMHO maintainers should reach an agreement to some extent on how
> >>> compatibility should be achieved. I just found silly to add a property
> >>> to fix a real bug in the model, and we preserve the bug all over
> >>> releases.
> >>
> >> That's the price for the stable ABI. See one of my recent fix -
> >> d83f46d189 virtio-pci: compat page aligned ATS. It keeps the "buggy"
> >> behavior to unbreak the migration.
> >>
> > But this series does not break the migration, as we discussed in the
> > previous thread.
>
>
> It actually did,
>
> (qemu) qemu-kvm: get_pci_config_device: Bad config data: i=0x104 read:
>      0 device: 20 cmask: ff wmask: 0 w1cmask:0
>
> Since the register is RO.

Did you mean the VET register is read-only? I don't understand this as
the manual says it's RW and Linux driver programs it to 0x8100.

>
>
> >
> >>> I can find plenty of examples in the current QEMU tree that were
> >>> accepted that changed the bugous register behavior, but it was not
> >>> asked to add new properties to keep the bugos behavior.
> >>>
> >>> e.g.: commit ce8e43760e8e ("hw/net: fsl_etsec: Reverse the RCTRL.RSF logic")
> >>
> >> I guess it's simply because fsl_etsec is not used in any
> >> distributions/production environments or the maintainer may just not
> >> notice things like this.
> >>
> >> But for e1000(e), we should stick to a stable ABI for consistency.
> >> Otherwise it would be very tricky to fix them after we saw real issues.
> >> We had learnt a lot during the past decade.
> >>
> > Okay, do we have such a kind of widely used device model list? And we
> > should document such a process that we should keep compatibility on
> > these devices as well.
>
>
> I can only say for networking devices:
>
> - e1000(e)
> - rtl8139
> - virtio-net
>

Please, at least have these documented as a development process.

> >
> > Regarding this VET register, do you know what guest relies on the POR
> > value which is zero?
>
>
> I don't know.
>
>
> > Zero is not a valid ethernet VLAN type. I don't
> > think changing this will break any guests.
>
>
> You might be right here, but it would be late if we find it breaks any one.
>
> If it's not a lot of work, I tend to bother with compat stuffs for this.
>

Regards,
Bin