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

From: Jason Wang
Subject: Re: [PATCH v2 1/3] hw/net: e1000: Correct the initial value of VET register
Date: Wed, 14 Jul 2021 11:10:30 +0800
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

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 |
+    [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.
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
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

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.

It should compat capareg for the old value for old machine types.

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.



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