From: Stefano Stabellini <sstabellini@kernel.org>
To: Julien Grall <julien@xen.org>
Cc: xen-devel@lists.xenproject.org,
Stefano Stabellini <sstabellini@kernel.org>,
Volodymyr_Babchuk@epam.com,
Stefano Stabellini <stefano.stabellini@xilinx.com>
Subject: Re: [PATCH 11/12] xen/arm: if xen_force don't try to setup the IOMMU
Date: Thu, 30 Apr 2020 18:25:34 -0700 (PDT) [thread overview]
Message-ID: <alpine.DEB.2.21.2004301249490.28941@sstabellini-ThinkPad-T480s> (raw)
In-Reply-To: <b60d6ae3-e300-04a1-a884-e73d01a108d5@xen.org>
On Thu, 30 Apr 2020, Julien Grall wrote:
> Hi Stefano,
>
> On 29/04/2020 22:55, Stefano Stabellini wrote:
> > On Wed, 15 Apr 2020, Julien Grall wrote:
> > > Hi Stefano,
> > >
> > > On 15/04/2020 02:02, Stefano Stabellini wrote:
> > > > If xen_force (which means xen,force-assign-without-iommu was requested)
> > > > don't try to add the device to the IOMMU. Return early instead.
> > >
> > >
> > > Could you explain why this is an issue to call xen_force after
> > > iommu_add_dt_device()?
> >
> > There are two issues. I should add info about both of them to the commit
> > message.
> >
> >
> > The first issue is that an error returned by iommu_add_dt_device (for
> > any reason) would cause handle_passthrough_prop to stop and return error
> > right away. But actually the iommu is not needed for that device if
> > xen_force is set.
>
> During boot, Xen will configure the IOMMUs to fault on any DMA transactions
> that are not valid. So if you don't call iommu_assign_dt_device(), then your
> device will be unusable.
>
> Without your patch, the user will know directly something went wrong. With
> your patch, the fault may occur much later and be more difficult to
> diagnostics.
>
> > (In fact, one of the reasons why a user might want to set
> > force-assign-without-iommu is because there are iommu issues with a
> > device.)
> This would not work because of the reasons I explained above. The only way
> would be to configure the IOMMU in bypass mode for that device.
>
> So you would still need to call the IOMMU subsystem.
What you wrote makes sense and also matches my understanding. But some
of my testing results confused me. I am going to go back and look at
this closely again, but I am thinking of dropping this patch and
avoiding interfering with IOMMU settings.
next prev parent reply other threads:[~2020-05-01 1:25 UTC|newest]
Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-15 1:02 [PATCH 0/12] direct-map DomUs Stefano Stabellini
2020-04-15 1:02 ` [PATCH 01/12] xen: introduce xen_dom_flags Stefano Stabellini
2020-04-15 9:12 ` Jan Beulich
2020-04-15 13:26 ` Julien Grall
2020-04-29 23:57 ` Stefano Stabellini
2020-04-15 1:02 ` [PATCH 02/12] xen/arm: introduce arch_xen_dom_flags and direct_map Stefano Stabellini
2020-04-15 10:27 ` Jan Beulich
2020-04-15 11:27 ` Andrew Cooper
2020-04-30 0:34 ` Stefano Stabellini
2020-04-15 1:02 ` [PATCH 03/12] xen/arm: introduce 1:1 mapping for domUs Stefano Stabellini
2020-04-15 13:36 ` Julien Grall
2020-05-01 1:26 ` Stefano Stabellini
2020-05-01 8:30 ` Julien Grall
2020-05-09 0:07 ` Stefano Stabellini
2020-05-09 9:56 ` Julien Grall
2020-04-15 1:02 ` [PATCH 04/12] xen: split alloc_heap_pages in two halves for reusability Stefano Stabellini
2020-04-15 11:22 ` Wei Liu
2020-04-17 10:02 ` Jan Beulich
2020-04-29 23:09 ` Stefano Stabellini
2020-04-15 1:02 ` [PATCH 05/12] xen: introduce reserve_heap_pages Stefano Stabellini
2020-04-15 13:24 ` Julien Grall
2020-04-17 10:11 ` Jan Beulich
2020-04-29 22:46 ` Stefano Stabellini
2020-04-30 6:29 ` Jan Beulich
2020-04-30 16:21 ` Stefano Stabellini
2020-05-04 9:16 ` Jan Beulich
2020-04-30 14:51 ` Julien Grall
2020-04-30 17:00 ` Stefano Stabellini
2020-04-30 18:27 ` Julien Grall
2020-05-12 1:10 ` Stefano Stabellini
2020-05-12 8:57 ` Julien Grall
2020-04-15 1:02 ` [PATCH 06/12] xen/arm: reserve 1:1 memory for direct_map domUs Stefano Stabellini
2020-04-15 13:38 ` Julien Grall
2020-04-15 1:02 ` [PATCH 07/12] xen/arm: new vgic: rename vgic_cpu/dist_base to c/dbase Stefano Stabellini
2020-04-15 13:41 ` Julien Grall
2020-04-15 1:02 ` [PATCH 08/12] xen/arm: if is_domain_direct_mapped use native addresses for GICv2 Stefano Stabellini
2020-04-15 14:00 ` Julien Grall
2020-05-01 1:26 ` Stefano Stabellini
2020-05-01 8:23 ` Julien Grall
2020-05-09 0:06 ` Stefano Stabellini
2020-04-15 1:02 ` [PATCH 09/12] xen/arm: if is_domain_direct_mapped use native addresses for GICv3 Stefano Stabellini
2020-04-15 14:09 ` Julien Grall
2020-05-01 1:31 ` Stefano Stabellini
2020-05-01 8:40 ` Julien Grall
2020-05-09 0:06 ` Stefano Stabellini
2020-04-15 1:02 ` [PATCH 10/12] xen/arm: if is_domain_direct_mapped use native UART address for vPL011 Stefano Stabellini
2020-04-15 14:11 ` Julien Grall
2020-05-01 1:26 ` Stefano Stabellini
2020-05-01 8:09 ` Julien Grall
2020-05-09 0:07 ` Stefano Stabellini
2020-05-09 10:11 ` Julien Grall
2020-05-11 22:58 ` Stefano Stabellini
2020-04-15 1:02 ` [PATCH 11/12] xen/arm: if xen_force don't try to setup the IOMMU Stefano Stabellini
2020-04-15 14:12 ` Julien Grall
2020-04-29 21:55 ` Stefano Stabellini
2020-04-30 13:51 ` Julien Grall
2020-05-01 1:25 ` Stefano Stabellini [this message]
2020-04-15 1:02 ` [PATCH 12/12] xen/arm: call iomem_permit_access for passthrough devices Stefano Stabellini
2020-04-15 14:18 ` Julien Grall
2020-04-29 20:47 ` Stefano Stabellini
2020-04-30 13:01 ` Julien Grall
2020-05-24 14:12 ` Julien Grall
2020-05-26 16:46 ` Stefano Stabellini
2020-05-27 18:09 ` Julien Grall
2020-04-16 8:59 ` [PATCH 0/12] direct-map DomUs Julien Grall
2020-04-29 20:16 ` Stefano Stabellini
2020-04-30 12:54 ` Julien Grall
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