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Subject: Re: [RFC 1/2] vhost: IFC VF hardware operation layer
To:     Simon Horman <simon.horman@netronome.com>,
        "Zhu, Lingshan" <lingshan.zhu@intel.com>
Cc:     mst@redhat.com, alex.williamson@redhat.com,
        linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, kvm@vger.kernel.org,
        netdev@vger.kernel.org, dan.daly@intel.com,
        cunming.liang@intel.com, tiwei.bie@intel.com, jason.zeng@intel.com,
        zhiyuan.lv@intel.com
References: <20191016011041.3441-1-lingshan.zhu@intel.com>
 <20191016011041.3441-2-lingshan.zhu@intel.com>
 <20191016095347.5sb43knc7eq44ivo@netronome.com>
 <075be045-3a02-e7d8-672f-4a207c410ee8@intel.com>
 <20191021163139.GC4486@netronome.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <15d94e61-9b3d-7854-b65e-6fea6db75450@redhat.com>
Date:   Tue, 22 Oct 2019 09:32:36 +0800
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On 2019/10/22 =E4=B8=8A=E5=8D=8812:31, Simon Horman wrote:
> On Mon, Oct 21, 2019 at 05:55:33PM +0800, Zhu, Lingshan wrote:
>> On 10/16/2019 5:53 PM, Simon Horman wrote:
>>> Hi Zhu,
>>>
>>> thanks for your patch.
>>>
>>> On Wed, Oct 16, 2019 at 09:10:40AM +0800, Zhu Lingshan wrote:
> ...
>
>>>> +static void ifcvf_read_dev_config(struct ifcvf_hw *hw, u64 offset,
>>>> +=09=09       void *dst, int length)
>>>> +{
>>>> +=09int i;
>>>> +=09u8 *p;
>>>> +=09u8 old_gen, new_gen;
>>>> +
>>>> +=09do {
>>>> +=09=09old_gen =3D ioread8(&hw->common_cfg->config_generation);
>>>> +
>>>> +=09=09p =3D dst;
>>>> +=09=09for (i =3D 0; i < length; i++)
>>>> +=09=09=09*p++ =3D ioread8((u8 *)hw->dev_cfg + offset + i);
>>>> +
>>>> +=09=09new_gen =3D ioread8(&hw->common_cfg->config_generation);
>>>> +=09} while (old_gen !=3D new_gen);
>>> Would it be wise to limit the number of iterations of the loop above?
>> Thanks but I don't quite get it. This is used to make sure the function
>> would get the latest config.
> I am worried about the possibility that it will loop forever.
> Could that happen?
>
> ...


My understanding is that the function here is similar to virtio config=20
generation [1]. So this can only happen for a buggy hardware.

Thanks

[1]=20
https://docs.oasis-open.org/virtio/virtio/v1.1/csprd01/virtio-v1.1-csprd01.=
html=20
Section 2.4.1


>
>>>> +static void io_write64_twopart(u64 val, u32 *lo, u32 *hi)
>>>> +{
>>>> +=09iowrite32(val & ((1ULL << 32) - 1), lo);
>>>> +=09iowrite32(val >> 32, hi);
>>>> +}
>>> I see this macro is also in virtio_pci_modern.c
>>>
>>> Assuming lo and hi aren't guaranteed to be sequential
>>> and thus iowrite64_hi_lo() cannot be used perhaps
>>> it would be good to add a common helper somewhere.
>> Thanks, I will try after this IFC patchwork, I will cc you.
> Thanks.
>
> ...