linux-kernel - Re: [PATCH] optee: don't fail on unsuccessful device enumeration

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Date:   Thu, 14 May 2020 04:18:33 +0300
From:   Volodymyr Babchuk <vlad.babchuk@...il.com>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     Jens Wiklander <jens.wiklander@...aro.org>,
        open list <linux-kernel@...r.kernel.org>,
        "tee-dev @ lists . linaro . org" <tee-dev@...ts.linaro.org>,
        Jerome Forissier <jerome@...issier.org>
Subject: Re: [PATCH] optee: don't fail on unsuccessful device enumeration

Hi Sumit,

On Wed, 13 May 2020 at 11:24, Sumit Garg <sumit.garg@...aro.org> wrote:
>
> Hi Volodymyr,
>
> On Wed, 13 May 2020 at 13:30, Jens Wiklander <jens.wiklander@...aro.org> wrote:
> >
> > Hi Volodymyr,
> >
> > On Wed, May 13, 2020 at 2:36 AM Volodymyr Babchuk
> > <vlad.babchuk@...il.com> wrote:
> > >
> > > optee_enumerate_devices() can fail for multiple of reasons. For
> > > example, I encountered issue when Xen OP-TEE mediator NACKed
> > > PTA_CMD_GET_DEVICES call.
>
> Could you share a detailed description of the issue which you are
> facing? optee_enumerate_devices() is a simple invocation of pseudo TA
> and cases where OP-TEE doesn't provide corresponding pseudo TA are
> handled very well.

Yes, I did some research and looks like issue is broader, than I
expected.  It is my fault, that I wasn't paying attention to the tee
client support in the kernel.  Basically, it is incompatible with the
virtualization feature. You see, the main issue with virtual machines
is the second stage MMU. Intermediate physical address, that appear to
be contiguous for the kernel may be not contiguous in the real
physical memory due to 2nd stage MMU mappings. This is the reason I
introduced OPTEE_MSG_ATTR_NONCONTIG in the kernel driver.

But, looks like kernel-side optee client does not use this feature. It
tries to provide SHM buffer as a simple contiguous span of memory. Xen
blocks calls with OPTEE_MSG_ATTR_TYPE_TMEM_*   but without
OPTEE_MSG_ATTR_NONCONTIG , because it can't translate IPAs to PAs for
such buffers. This is why call to  PTA_CMD_GET_DEVICES fails.

Valid fix would be to use OPTEE_MSG_ATTR_NONCONTIG whenever possible.

>
> > > This should not result in driver
> > > initialization error because this is an optional feature.
>
> I wouldn't call it an optional feature as there might be real kernel
> drivers dependent on this enumeration. Also, it is a simple example to
> self test OP-TEE functionality too. So I am not sure how much
> functional OP-TEE would be if this basic TA invocation fails.

Well, it fixed case when Xen is involved. I think, this is valid
combination, when platform have the newest OP-TEE, but slightly older
kernel. So, imagine that OP-TEE provides PTA_CMD_GET_DEVICES, but
kernel can't use because it uses plain TMEM arguments,which are not
supported in virtualized environment.

If there are kernel drivers, that depend on this PTA, they would not
work in any case. But at least userspace clients still be able to use
OP-TEE. This is why I call this feature "optional".

-- 
WBR Volodymyr Babchuk aka lorc [+380976646013]
mailto: vlad.babchuk@...il.com