From: Manivannan Sadhasivam <mani@kernel.org>
To: Shunsuke Mie <mie@igel.co.jp>
Cc: linux-pci@vger.kernel.org, virtualization@lists.linux.dev,
mst@redhat.com, jasowang@redhat.com, xuanzhuo@linux.alibaba.com
Subject: Re: [RFC] Legacy Virtio Driver with Device Has Limited Memory Access
Date: Thu, 16 May 2024 14:59:13 +0200 [thread overview]
Message-ID: <20240516125913.GC11261@thinkpad> (raw)
In-Reply-To: <CANXvt5r00Y5VGKSFXFnwbvGF+fhh2uNvU5VBGwECA9yabK4=Uw@mail.gmail.com>
On Thu, May 16, 2024 at 01:38:40PM +0900, Shunsuke Mie wrote:
> Hi virtio folks,
>
You forgot to CC the actual Virtio folks. I've CCed them now.
> I'm writing to discuss finding a workaround with Virtio drivers and legacy
> devices with limited memory access.
>
> # Background
> The Virtio specification defines a feature (VIRTIO_F_ACCESS_PLATFORM) to
> indicate devices requiring restricted memory access or IOMMU translation. This
> feature bit resides at position 33 in the 64-bit Features register on modern
> interfaces. When the linux virtio driver finds the flag, the driver uses DMA
> API that handles to use of appropriate memory.
>
> # Problem
> However, legacy devices only have a 32-bit register for the features bits.
> Consequently, these devices cannot represent the ACCESS_PLATFORM bit. As a
> result, legacy devices with restricted memory access cannot function
> properly[1]. This is a legacy spec issue, but I'd like to find a workaround.
>
> # Proposed Solutions
> I know these are not ideal, but I propose the following solution.
> Driver-side:
> - Implement special handling similar to xen_domain.
> In xen_domain, linux virtio driver enables to use the DMA API.
> - Introduce a CONFIG option to adjust the DMA API behavior.
> Device-side:
> Due to indistinguishability from the guest's perspective, a device-side
> solution might be difficult.
>
> I'm open to any comments or suggestions you may have on this issue or
> alternative approaches.
>
> [1] virtio-net PCI endpoint function using PCIe Endpoint Framework,
> https://lore.kernel.org/lkml/54ee46c3-c845-3df3-8ba0-0ee79a2acab1@igel.co.jp/t/
> The Linux PCIe endpoint framework is used to implement the virtio-net device on
> a legacy interface. This is necessary because of the framework and hardware
> limitation.
>
We can fix the endpoint framework limitation, but the problem lies with some
platforms where we cannot write to vendor capability registers and still have
IOMMU.
- Mani
--
மணிவண்ணன் சதாசிவம்
next prev parent reply other threads:[~2024-05-16 12:59 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-16 4:38 [RFC] Legacy Virtio Driver with Device Has Limited Memory Access Shunsuke Mie
2024-05-16 12:59 ` Manivannan Sadhasivam [this message]
2024-05-16 13:13 ` Michael S. Tsirkin
2024-05-20 10:24 ` Shunsuke Mie
2024-05-30 15:07 ` Manivannan Sadhasivam
2024-05-20 13:22 ` Michael S. Tsirkin
2024-05-30 16:43 ` Michael S. Tsirkin
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20240516125913.GC11261@thinkpad \
--to=mani@kernel.org \
--cc=jasowang@redhat.com \
--cc=linux-pci@vger.kernel.org \
--cc=mie@igel.co.jp \
--cc=mst@redhat.com \
--cc=virtualization@lists.linux.dev \
--cc=xuanzhuo@linux.alibaba.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).