From: Thiago Jung Bauermann <bauerman@linux.ibm.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Mike Anderson <andmike@linux.ibm.com>,
Michael Roth <mdroth@linux.vnet.ibm.com>,
Jean-Philippe Brucker <jean-philippe.brucker@arm.com>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Jason Wang <jasowang@redhat.com>,
Alexey Kardashevskiy <aik@linux.ibm.com>,
Ram Pai <linuxram@us.ibm.com>,
linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org,
Paul Mackerras <paulus@ozlabs.org>,
iommu@lists.linux-foundation.org, linuxppc-dev@lists.ozlabs.org,
Christoph Hellwig <hch@lst.de>,
David Gibson <david@gibson.dropbear.id.au>
Subject: Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted
Date: Thu, 27 Jun 2019 22:58:40 -0300 [thread overview]
Message-ID: <877e96qxm7.fsf@morokweng.localdomain> (raw)
In-Reply-To: <20190603211528-mutt-send-email-mst@kernel.org>
Michael S. Tsirkin <mst@redhat.com> writes:
> On Mon, Jun 03, 2019 at 10:13:59PM -0300, Thiago Jung Bauermann wrote:
>>
>>
>> Michael S. Tsirkin <mst@redhat.com> writes:
>>
>> > On Wed, Apr 17, 2019 at 06:42:00PM -0300, Thiago Jung Bauermann wrote:
>> >> I rephrased it in terms of address translation. What do you think of
>> >> this version? The flag name is slightly different too:
>> >>
>> >>
>> >> VIRTIO_F_ACCESS_PLATFORM_NO_TRANSLATION This feature has the same
>> >> meaning as VIRTIO_F_ACCESS_PLATFORM both when set and when not set,
>> >> with the exception that address translation is guaranteed to be
>> >> unnecessary when accessing memory addresses supplied to the device
>> >> by the driver. Which is to say, the device will always use physical
>> >> addresses matching addresses used by the driver (typically meaning
>> >> physical addresses used by the CPU) and not translated further. This
>> >> flag should be set by the guest if offered, but to allow for
>> >> backward-compatibility device implementations allow for it to be
>> >> left unset by the guest. It is an error to set both this flag and
>> >> VIRTIO_F_ACCESS_PLATFORM.
>> >
>> >
>> > OK so VIRTIO_F_ACCESS_PLATFORM is designed to allow unpriveledged
>> > drivers. This is why devices fail when it's not negotiated.
>>
>> Just to clarify, what do you mean by unprivileged drivers? Is it drivers
>> implemented in guest userspace such as with VFIO? Or unprivileged in
>> some other sense such as needing to use bounce buffers for some reason?
>
> I had drivers in guest userspace in mind.
Great. Thanks for clarifying.
I don't think this flag would work for guest userspace drivers. Should I
add a note about that in the flag definition?
>> > This confuses me.
>> > If driver is unpriveledged then what happens with this flag?
>> > It can supply any address it wants. Will that corrupt kernel
>> > memory?
>>
>> Not needing address translation doesn't necessarily mean that there's no
>> IOMMU. On powerpc we don't use VIRTIO_F_ACCESS_PLATFORM but there's
>> always an IOMMU present. And we also support VFIO drivers. The VFIO API
>> for pseries (sPAPR section in Documentation/vfio.txt) has extra ioctls
>> to program the IOMMU.
>>
>> For our use case, we don't need address translation because we set up an
>> identity mapping in the IOMMU so that the device can use guest physical
>> addresses.
>
> And can it access any guest physical address?
Sorry, I was mistaken. We do support VFIO in guests but not for virtio
devices, only for regular PCI devices. In which case they will use
address translation.
>> If the guest kernel is concerned that an unprivileged driver could
>> jeopardize its integrity it should not negotiate this feature flag.
>
> Unfortunately flag negotiation is done through config space
> and so can be overwritten by the driver.
Ok, so the guest kernel has to forbid VFIO access on devices where this
flag is advertised.
>> Perhaps there should be a note about this in the flag definition? This
>> concern is platform-dependant though. I don't believe it's an issue in
>> pseries.
>
> Again ACCESS_PLATFORM has a pretty open definition. It does actually
> say it's all up to the platform.
>
> Specifically how will VIRTIO_F_ACCESS_PLATFORM_NO_TRANSLATION be
> implemented portably? virtio has no portable way to know
> whether DMA API bypasses translation.
The fact that VIRTIO_F_ACCESS_PLATFORM_NO_TRANSLATION is set
communicates that knowledge to virtio. There is a shared understanding
between the guest and the host about what this flag being set means.
--
Thiago Jung Bauermann
IBM Linux Technology Center
_______________________________________________
iommu mailing list
iommu@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/iommu
next prev parent reply other threads:[~2019-06-28 1:59 UTC|newest]
Thread overview: 64+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-29 17:08 [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted Thiago Jung Bauermann
2019-01-29 17:42 ` Thiago Jung Bauermann
2019-01-29 19:02 ` Michael S. Tsirkin
2019-01-30 2:24 ` Jason Wang
2019-01-30 2:36 ` Michael S. Tsirkin
2019-01-30 3:05 ` Jason Wang
2019-01-30 3:26 ` Michael S. Tsirkin
2019-01-30 7:44 ` Christoph Hellwig
2019-02-04 18:15 ` Thiago Jung Bauermann
2019-02-04 21:38 ` Michael S. Tsirkin
2019-02-05 7:24 ` Christoph Hellwig
[not found] ` <20190205072407.GA4311-jcswGhMUV9g@public.gmane.org>
2019-02-05 16:13 ` Michael S. Tsirkin
2019-03-26 16:53 ` Michael S. Tsirkin
2019-02-04 18:14 ` Thiago Jung Bauermann
2019-02-04 20:23 ` Michael S. Tsirkin
2019-03-20 16:13 ` Thiago Jung Bauermann
2019-03-20 21:17 ` Michael S. Tsirkin
2019-03-22 0:05 ` Thiago Jung Bauermann
2019-03-23 21:01 ` Michael S. Tsirkin
2019-03-25 0:57 ` David Gibson
2019-04-17 21:42 ` Thiago Jung Bauermann
2019-04-17 21:42 ` Thiago Jung Bauermann
[not found] ` <20190323165456-mutt-send-email-mst-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
2019-04-17 21:42 ` Thiago Jung Bauermann
2019-04-17 21:42 ` Thiago Jung Bauermann
2019-04-19 23:09 ` Michael S. Tsirkin
2019-04-19 23:09 ` Michael S. Tsirkin
2019-04-25 1:01 ` Thiago Jung Bauermann
2019-04-25 1:01 ` Thiago Jung Bauermann
[not found] ` <875zr228zf.fsf-wxVGo8vDogbJvNEK5ZsId7p2dZbC/Bob@public.gmane.org>
2019-04-25 1:18 ` Michael S. Tsirkin
2019-04-25 1:18 ` Michael S. Tsirkin
2019-04-26 23:56 ` Thiago Jung Bauermann
2019-04-26 23:56 ` Thiago Jung Bauermann
2019-05-20 13:08 ` Michael S. Tsirkin
2019-05-20 13:16 ` Michael S. Tsirkin
2019-06-04 1:13 ` Thiago Jung Bauermann
2019-06-04 1:42 ` Michael S. Tsirkin
2019-06-28 1:58 ` Thiago Jung Bauermann [this message]
2019-07-01 14:17 ` Michael S. Tsirkin
2019-07-14 5:51 ` Thiago Jung Bauermann
2019-07-15 14:35 ` Michael S. Tsirkin
2019-07-15 20:29 ` Thiago Jung Bauermann
2019-07-15 20:36 ` Michael S. Tsirkin
2019-07-15 22:03 ` Thiago Jung Bauermann
2019-07-15 22:16 ` Michael S. Tsirkin
2019-07-15 23:05 ` Thiago Jung Bauermann
2019-07-15 23:24 ` Benjamin Herrenschmidt
2019-07-18 3:39 ` Thiago Jung Bauermann
2019-08-10 18:57 ` Michael S. Tsirkin
2019-08-10 22:07 ` Ram Pai
2019-08-11 5:56 ` Christoph Hellwig
2019-08-11 6:46 ` Ram Pai
2019-08-11 8:44 ` Michael S. Tsirkin
2019-08-12 12:13 ` Christoph Hellwig
2019-08-12 20:29 ` Ram Pai
2019-08-11 8:42 ` Michael S. Tsirkin
2019-08-11 8:55 ` Michael S. Tsirkin
2019-08-12 12:15 ` Christoph Hellwig
2019-09-06 5:07 ` Michael S. Tsirkin
2019-08-12 9:51 ` David Gibson
2019-08-13 13:26 ` Christoph Hellwig
2019-08-13 14:24 ` David Gibson
2019-08-13 15:45 ` Ram Pai
2019-08-26 17:48 ` Ram Pai
2019-08-11 8:12 ` Michael S. Tsirkin
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