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Message-ID: <ea6ecd2b-5391-3820-d3fd-411b60a5a2ec@oracle.com>
Date: Tue, 15 Aug 2023 15:30:46 -0700
From: Si-Wei Liu <si-wei.liu@...cle.com>
To: Jason Wang <jasowang@...hat.com>
Cc: eperezma@...hat.com, gal@...dia.com, linux-kernel@...r.kernel.org,
mst@...hat.com, virtualization@...ts.linux-foundation.org,
xuanzhuo@...ux.alibaba.com
Subject: Re: [PATCH RFC 4/4] vhost-vdpa: introduce IOTLB_PERSIST backend
feature bit
On 8/14/2023 7:25 PM, Jason Wang wrote:
> On Tue, Aug 15, 2023 at 9:45 AM Si-Wei Liu <si-wei.liu@...cle.com> wrote:
>> Signed-off-by: Si-Wei Liu <si-wei.liu@...cle.com>
>> ---
>> drivers/vhost/vdpa.c | 16 +++++++++++++++-
>> include/uapi/linux/vhost_types.h | 2 ++
>> 2 files changed, 17 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
>> index 62b0a01..75092a7 100644
>> --- a/drivers/vhost/vdpa.c
>> +++ b/drivers/vhost/vdpa.c
>> @@ -406,6 +406,14 @@ static bool vhost_vdpa_can_resume(const struct vhost_vdpa *v)
>> return ops->resume;
>> }
>>
>> +static bool vhost_vdpa_has_persistent_map(const struct vhost_vdpa *v)
>> +{
>> + struct vdpa_device *vdpa = v->vdpa;
>> + const struct vdpa_config_ops *ops = vdpa->config;
>> +
>> + return (!ops->set_map && !ops->dma_map) || ops->reset_map;
> So this means the IOTLB/IOMMU mappings have already been decoupled
> from the vdpa reset.
Not in the sense of API, it' been coupled since day one from the
implementations of every on-chip IOMMU parent driver, namely mlx5_vdpa
and vdpa_sim. Because of that, later on the (improper) support for
virtio-vdpa, from commit 6f5312f80183 ("vdpa/mlx5: Add support for
running with virtio_vdpa") and 6c3d329e6486 ("vdpa_sim: get rid of DMA
ops") misused the .reset() op to realize 1:1 mapping, rendering strong
coupling between device reset and reset of iotlb mappings. This series
try to rectify that implementation deficiency, while keep userspace
continuing to work with older kernel behavior.
> So it should have been noticed by the userspace.
Yes, userspace had noticed this no-chip IOMMU discrepancy since day one
I suppose. Unfortunately there's already code in userspace with this
assumption in mind that proactively tears down and sets up iotlb mapping
around vdpa device reset...
> I guess we can just fix the simulator and mlx5 then we are fine?
Only IF we don't care about running new QEMU on older kernels with
flawed on-chip iommu behavior around reset. But that's a big IF...
Regards,
-Siwei
>
> Thanks
>
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