Re: [Qemu-devel] [Qemu-arm] [PATCH v2 2/2] arm_gicv3_kvm: kvm_dist_get/put: skip the registers banked by GICR

[Peter Maydell <peter.maydell@linaro.org>]

On 20 March 2018 at 11:36, Shannon Zhao <zhaoshenglong@huawei.com> wrote:
>
>
> On 2018/3/20 19:22, Peter Maydell wrote:
>> On 20 March 2018 at 07:26, Shannon Zhao <zhaoshenglong@huawei.com> wrote:
>>> While we skip the GIC_INTERNAL irqs, we don't change the register offset
>>> accordingly. This will overlap the GICR registers value and leave the
>>> last GIC_INTERNAL irq's registers out of update.
>>>
>>> Fix this by skipping the registers banked by GICR.
>>>
>>
>> I'm still not entirely sure what the underlying problem
>> you're trying to fix is...
>>
>> Do we fail to correctly migrate a VM without this change?
>> Does the code work on some host CPU/GIC implementations but
>> not others? Is this just improving efficiency by avoiding
>> doing some unnecessary work?
>>
> When we reboot a VM and before entering uefi or guest kernel, we expect
> all these registers staying at the initial state. But currently these
> registers of the last 32 irqs are not reset. For example, the PRIORITY
> of irq from 32 to 255 is 0 but the PRIORITY of irq from 256 to 287 is
> 0xa0(Linux kernel set the PRIORITY to 0xa0 by default).
>
> When migrating a VM, since we don't save and restore the registers of
> the last 32 irq, so the PRIORITY is 0 while we expecting 0xa0.
> And also it will overlap the PRIORITY of SGIs and PPIs.
>
> We don't fail to migrate a vm since currently we don't use the last 32
> irqs in virt machine. But the bug is still there.

Oh, I see, the number of registers we transfer is accounting
for the first N registers in the bank not being used, but the
first register offset to transfer wasn't.

Can you still successfully migrate a VM from a QEMU version
without this bugfix to one with the bugfix ?

thanks
-- PMM