linux-kernel - Re: [PATCH V4 0/8] virtio: Solution to restrict memory access under Xen using xen-grant DMA-mapping layer

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Message-ID: <CAOh2x=kxpdisV+tqcYOoZGSKA8YjPMej+7u19Jpa1jmVcZCaxA@mail.gmail.com>
Date:   Wed, 15 Jun 2022 11:53:54 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Oleksandr Tyshchenko <olekstysh@...il.com>
Cc:     xen-devel@...ts.xenproject.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        virtualization@...ts.linux-foundation.org, x86@...nel.org,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>, Julien Grall <julien@....org>,
        Bertrand Marquis <bertrand.marquis@....com>,
        Wei Chen <Wei.Chen@....com>, Henry Wang <Henry.Wang@....com>,
        Kaly Xin <Kaly.Xin@....com>, Jiamei Xie <Jiamei.Xie@....com>,
        Alex Bennée <alex.bennee@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>
Subject: Re: [PATCH V4 0/8] virtio: Solution to restrict memory access under
 Xen using xen-grant DMA-mapping layer

Hi Oleksandr,

On Mon, Jun 6, 2022 at 10:16 AM Oleksandr Tyshchenko
<olekstysh@...il.com> wrote:
> The high level idea is to create new Xen’s grant table based DMA-mapping layer for the guest Linux whose main
> purpose is to provide a special 64-bit DMA address which is formed by using the grant reference (for a page
> to be shared with the backend) with offset and setting the highest address bit (this is for the backend to
> be able to distinguish grant ref based DMA address from normal GPA). For this to work we need the ability
> to allocate contiguous (consecutive) grant references for multi-page allocations. And the backend then needs
> to offer VIRTIO_F_ACCESS_PLATFORM and VIRTIO_F_VERSION_1 feature bits (it must support virtio-mmio modern
> transport for 64-bit addresses in the virtqueue).

I was trying your series, from Linus's tree now and started seeing
boot failures,
failed to mount rootfs. And the reason probably is these messages:

[ 1.222498] virtio_scsi virtio1: device must provide VIRTIO_F_ACCESS_PLATFORM
[ 1.316334] virtio_net virtio0: device must provide VIRTIO_F_ACCESS_PLATFORM

I understand from your email that the backends need to offer
VIRTIO_F_ACCESS_PLATFORM flag now, but should this requirement be a
bit soft ? I mean shouldn't we allow both types of backends to run with the same
kernel, ones that offer this feature and others that don't ? The ones that don't
offer the feature, should continue to work like they used to, i.e.
without the restricted
memory access feature.

I am testing Xen currently with help of Qemu  over my x86 desktop and
these backends
(scsi and net) are part of QEMU itself I think, and I don't really
want to go and make the
change there.

Thanks.

--
Viresh